The garden and courtyard have been decorated with red and white bunting. Just outside of the walls of Skyhold, there is a Summerpole/Maypole, which people can dance around. There's also a bonfire near it.
The main hall is decorated with the same bunting, with flowers on all of the tables. It's still kind of rustic, as there's not a lot that can be done this far from civilisation.
What's Happening
Summerday has a lot of similarities with May Day. So, I'm taking a few of the traditions from British May Day celebrations. Since, I know them best. Summerday celebrates the beginning of summer, new beginnings, and quite often marriage. It is celebrated as a holy day in Orlais. But, in most other countries, it's more of a time for revelry.
The festivities officially begin with a feast served mid-afternoon in the main hall. There is a wide selection of roast meats, potatoes and vegetables. As well as cheeses, bread etc. All served banquet style on the tables.
Once the food has been eaten, the festivities move outside. The garden is set up for the quiet contemplation and religious observances, for those that want them. Dancing is first available around the May/Summerpole, and then moves to the courtyard for jigs etc, accompanied by a group of minstrels, while Jaskier sulks that he's too injured to play.
As evening approaches, the bonfire outside of the walls is lit, and people can gather around for warmth, singing, storytelling, poetry and other entertainments. There will also be magical fireworks once darkness falls.
Drinks Menu
There's a makeshift bar in one corner, which has your standard fare. Although, cocktails aren't overly common in this world. There's a healthy amount of mulled drinks available from cider to wine, even mead.
However, there is a small table just to the side of the bar, which is clearly labeled to state that the drinks are magical in nature, and what they do. Because consent is important. These are:
May Day Rum Cocktail - Makes you very friendly for the next hour. Up to you whether this is platonic or romantic friendliness.