From some kind of official website:

Lamb Ale is the name given to an unusual yearly festival celebrated in the village of Kirtlington that dates back to the early 17th century.

Lamb Ale comes from the same school of religious feasts as Whitsun Ale, which was celebrated across the country on Whit Sunday, the seventh Sunday after Easter. At these events ale would be brewed and sold for the benefit of the local church and the parish as a whole.

The Lamb Ale at Kirtlington, on the other hand, was a far more elaborate affair that lasted for a full week of feasting, drinking, and dancing.

… which is why we are going. Even though it’s not a full week.