Dangerosa’s right, the only major difference between an Open House party and a regular one is that there’s no formal time you’re expected to arrive or to stay.
If the graduation party invitation reads 2-5, there’s a reason they want you there at 2, could be an activity or how the food will be served, whatever. Open Houses mean you can drop by at any time between the start and finish times listed.
Other types of parties can be Open House, like birthdays or housewarmings, but I think it is most common with graduation parties. In my area, the ‘burbs, graduating classes run near to a thousand kids per school. Nearly all of them will have graduation parties either commencement weekend or the couple of weeks after. Most want to attend their friends’ parties, so it makes more sense to leave things loose with the buffet style eating and informal timing.
Of course, it’s nice for the grownups too, I can put in an appearance at a co-worker’s kid’s graduation and not feel obligated to stay very long, etc.
Realtors use Open Houses to show prospective buyers, somehow we never get the two usages confused.
High school/college grads don’t get parties up north featherlou? Sucks to be you guys!!