Last fall we completely redid a section of our yard by killing the grass/weeds and re-sewing.This spring it has done really well. We recently had it aerated and overseeded. When would be the proper time to mow it again after the re-seeding? I do know you need to mow it high.

That's a tough question to answer.
What type of grass? If rye, it should be up and growing within 5-7 days, weather conditions permitting. Let it grow for a couple of weeks then mow.
If fescue or another type indigenous to your area, takes a while for some grasses to germinate.
What kind of mower will you be using? Power mower or tractor? Reel type mower? If the latter, anytime. If power, best wait or you'll just blow the seed away or risk the possibility of tearing the young grass out if your blades are dull.
For my locale reseeding now means waiting for weeks until mowing (6 week minimum).

For new grass, I always wait until it us about 3-4 inches before I mow. You want the new grass to be mature enough to withstand the mowing and not get ripped out of the ground. It is not so much a matter of how LONG it's been as how ready the grass is.

Thanks for your replies. It is a fescue blend of 3 tall fescues and we let it grow to about 3 inches but the original was taller already so we almost (not quite) let it grow too long. I cannot believe how much better it looks than the weeds and bare spots we had before.