My Preschool is year round. We have a regular school year that goes from September to May. (We essentially go from labor day to Memorial day.) Then we have a summer session from June through August. Because we usually say goodbye to the students going to Kindergarten in May we like to have a graduation ceremony for them. I did not do a graduation ceremony when I first started the preschool but a parent talked me into it about four years ago. Now I do it every year. Even though I don't have to, I know the parents and grandparents really love to celebrate their little people. We have held the graduation in several places. Our local Library, a community house, a covered pavilion in a large park, the botanic gardens...etc. We have done it during the day, afternoon and night. We always pick a theme, sing a song to the parents, and have a graduation march.
During May we also like to have an end of the year picnic for all the families. Not just the graduates. We always do a large potluck at a park with a play ground and spend time playing and chatting and enjoying each other company. It is also a great time for parents to finally meet and share numbers and make play dates for their kids in the upcoming summer months. They don't really move beyond a quick chat at pickups and drop offs, even though they know their kids are friends. This laid back end of year party always ends with exchanged numbers and new parent friendships.