My school - the one where I went and where I now teach - was 300 years old last year. Every year we celebrate Founders' Day on the first Thursday in March.
The day begins with a service at a church. Then the students go home (except those with impending exams) and the staff return to school.
In the evening the school is open so that former students can visit.
This year, the new headteacher, who was obviously delighted at being associated with a school that has such a history, discovered she had five Head Boys and three Head Girls at the evening do, so she quickly photographed and tweeted us.
I'm happy to report that the fat old bag has the excuse of being several decades older than the slimline younger versions...