There is not one instance in the lore of symbel being held outside, not a one. It was always done in a hall. Paul Bauschatz in his work The Well and the Tree has this to say on the rite of symbel:
Clearly the symbel was some kind of solemn occasion at which the participants significantly sat down. Within the rather strenuously active contexts of most Germanic texts, sitting suggests inaction, rest, and order. Order seems especially important, because to sit requires a place to sit, and a place suggests some apportioning of positions, and the apportioning suggests Urth. The symbel is also a joint activity; one never reads of someone at symbel alone. Those participating come together and sit, usually within a chieftain’s hall. The contexts are not explicit with respect to the location, most simply stating that such and-such people were sumbli at; however, the locations that are specified are inside, for example in Heorot, Hrothgar’s Hall, in Beowulf. There are no contexts in which it is explicitly stated that the symbel took place outside. (Paul Bautschatz The Well and the Tree
Like Bauschatz I can find no instances in the lore of symbel having ever taken place outside. It always took place in a hall. Stephen Pollington brings this up as well in his work The Mead-Hall. The reason for symbel taking place inside instead of outside is because of the significance of contained space according to Paul Bauschatz. Bauschatz says it better than I can:
If significant action is space-defining and if significant action is past-oriented, it does not seem at all unlikely that the structure of significant space would shape itself formally like that of the past. The emphasis upon weallas ‘walls’, natural or man-made, would suggest the edge of a defined space with respect to which significant action may occur. Spatial walls derive directly from the image of the well and its functional closure of activity. Such walls, whether of vehicles, as in the ritual of Nerthus, or those deriving from the mountains surrounding the pagan temple at Uppsala, define sacred spaces of particular importance. With respect to men’s actions alone, the hall of the chieftain provides the significant space; the symbel occurs within the hall. Beowulf’s own first battle with Grendel occurs within Hrothgar’s mead-hall. The importance of manmade halls, because they figure as a kind of container inside which significant events may occur, is central to all Germanic literature.(Paul Bauschatz, The Well and the Tree).
Essentially, the walls of the hall (or with modern Heathens, their home) are symbolic of the walls of the Well of Wyrd, the well in which all actions of the past are recorded. Since the aim of symbel is to get in touch with one’s wyrd, and through boasts, toasts, and oaths attempt to determine its flow, it is essential that the rite take place in contained space, the contained space being symbolic of the Well of Wyrd its self. The more links one can establish with the Well of Wyrd the better (the horn too according to Bauschatz is symbolic as the Well of Wyrd as well). Being outside there are no walls and as such no link to the Well of Wyrd.
This contained space can be seen also as an enclosure or innangarðs. The scholar Kirsten Hastrup has this to say on the concept of innangarðs.
When we turn to the layout of immediate space, it appears that the most significant distinction pertaining to the spacial arrangement of the farmstead was inni:úti (“inside:outside”). The borderline between the farmstead as centre and the world outside as periphery was drawn along the fence that surrounded the farm. The opposition between innangarðs and útangarðs (“inside” and “outside fence” respectively) had important socio-legal implications. (Kirsten Hastrup, Culture and History in Medieval Iceland)
She says further:
The important point is that in our period a structural and semantic opposition was operative between “inside” and “outside” the society-as-law, allowing for a merging of different kinds of beings in the conceptual “wild.” This anti-social space was inhabited by a whole range of spirits…landsvættir “spirits of the land,” huldufolk “hidden people,” jötnar “giants,” trölls “trolls,” and álfar “elves”…all of them belonged to the “wild” and it was partly against them that one had to defend ones-self… In this way the secure, well-known and personal innangards was symbolically separated from the dangerous unknown and nonhuman wild space outside the fence, útangards. (Kirsten Hastrup, Culture and History in Medieval Iceland )
So in addition to being symbolic of the Well of Wyrd, the structure in which one is holding symbel also serves as an enclosure, an innangarðs that serves to keep the “wilds” outside out. If done outside there may be many things spiritual and physical that may interfere with the rite of symbel. Done inside however, it is within the safety of a hall where much more can be controlled. An innangarðs is a realm of Man or the gods and is less likely to be meddled with by beings seeking to do harm or to interfere in the rite. These two reasons, the hall as symbolic of the Well of Wyrd, and the hall seen as an innangarðs are perhaps why symbel was always done inside in ancient times. Even if one does not accept Bauschatz’s theories on significant action in contained space, it is hard to deny Hastrup’s ideas on the enclosures of Man and the gods vs. that which is outside. Hastrup’s ideas would seem to be one basic to Heathenry.
So what if you are one of those groups that has always attempted to do the rite of “symbel” outside? Well, according to the reasons given here you were not performing symbel. For you to perform symbel you must be inside. Instead what you are performing is what Pollington refers to by the Old English word gebeorscipe which roughly translates as “drinking party.” Do not let the translation fool you. Gebeorscipe can be as every bit as serious as symbel in modern times. In fact, today it can take on the exact same form as symbel save it is being done outside. What makes gebeorscipe different if you accept Bauschatz’s ideas is that it does not require the enclosed space of symbel and as that is the case it does not accomplish the same things. Speech done in gebeorscipe is not significant speech. That is it is not speech that can alter one’s wyrd. There are exceptions to this such as swearing an oath on a sacred oath ring, but for the most part, speaking in gebeorscipe is no different than speaking at work, or in one’s home, at school, or any other non-ritual time. This is contrasted with symbel where every word you speak has significance as you are attempting to alter your wyrd, and have established a contained space with walls like that of the walls of the Well of Wyrd in which to do so. In symbel you boast of past great deeds in an effort to affect the outcomes of those deeds you plan to do and are vowing to do. In gebeorscipe while you may boast of past deeds and vow to do new ones, those past deeds will not necessarily affect the outcome of those deeds you are vowing to do. The reason quite simply is because you are not doing so in a contained space like the contained space of the Well of Wyrd.
That is not to say gebeorscipe does not serve a purpose. A Heathen community sitting around a fire swapping stories and tales, sharing drink together are getting things they may not even get out of symbel. They are bonding in such a way that is not easily done in other situations. There is something about sitting about a campfire making boasts and toasts and sharing with one’s family and friends that is not easily captured doing other things. Gebeorscipe serves a purpose nearly as important as symbel. But they are two different activities and serve different purposes. So if you have been performing “symbel” outside and truly do wish to do symbel, it is time to move it inside. On the other hand if togetherness and camaraderie are more important to you than altering your wyrd then by all means stay outside and do gebeorscipe. Myself, I would recommend doing both several times a year. There is much more that could be said on this, but I would suggest reading Paul Bauschatz’s The well and the Tree. Another good book concerning symbel is Pollington’s The Mead-Hall: Feasting in Anglo-Saxon England.
Bauchatz, Paul, The Well and the Tree: World and Time in Early Germanic Culture, University of Massachuetts Press; Amherst, 1982
Gronbech, Vilhelm, Culture of the Teutons, Oxford University Press; London, 1931
Hastrup, Kirsten, Culture and History in Medieval Iceland: An Anthropological Analysis of Structure and Change, Claredon Press; Oxford, 1985
Pollington, Stephen The Mead-Hall: The Feasting Tradition in Anglo-Saxon England Anglo-Saxon Books, Norfolk, 2003.