I had to do a little digging on this!

One thing to bear in mind with the dates, though, is that owing to confusion over the switch between Julian and Gregorian calendars, different lists of festivals can show the dates a month apart. On the Kemetic Orthodox version of the calendar (as it appears in Hemet's Daybook), it looks like this festival is in June rather than May. Sorry for any disappointment.

The Daybook gives this as "The Feast and Reception of Ra"; I'm guessing it's one of those festivals where the God is brought out of the shrine and all the people celebrate.