Whats the last date for buying a ticket for this?

I mat attend for the meeting with other guys off of here. At the moment though, I can't attend as it is my Grandmothers birthday that day (70), and as she brought me up, I feel obliged to be with her that day and take her for a meal.

Buuuuuut, she may yet book to see her friends in Greece as she has done in the poast, but she won't know until mid March. WOuld this be too late?