H.H.
you can have a graduation party anytime you want. I don't think there is a law that says it has to be that day or that weekend. My theory is a lot of people won't come to the actual graduation ceremony but will send a card and gift so over invite with the invitations. I have been to some that were 2 weeks after graduation as that is what worked for their family schedule. The guests coming to the ceremony from out of town may not come again for the party but maybe you could have a special dinner for them after the graduation. Even weddings have had receptions days or weeks after the wedding due to the ceremony being too small to hold all their friends and family or just wanting a separate reception to make it more special.