Personally, I try to go as long as possible to let the seedlings get rooted in and stronger. Usually about 4 weeks.

Yes, the yard looks bad in the interim.

Also, try to catch that first mow when the yard has dried out some. And, make sure your blade is as sharp as possible for that first mow and take it as slow and easy as possible. You might try to make as many turns as possible on sidewalks, paths, beyond the newly seeded areas, etc.

With the KBG in the mix, I'd wait longer than 2.5 weeks to mow. That's probably long enough to wait for fescue and/or rye, but KBG can take 3 weeks or more to germinate.

I'd wait as long as you can before mowing to give the KBG a chance.

wouldn't it be ok to mow prior to germination for the KBG since the seed would have washed down into the areation holes for the most part?

I will have the same problem as the original poster in a few weeks time.

I'm assuming the mower is a rotary mower. If that's the case, you'll suck any seeds that haven't germinated up and off the ground. And any that have germinated but not yet set roots well enough will be pulled up by the root.

Not trying to call out anyone, but he should have mowed his lawn low before seeding. It sounds like he may not have. For me, the lesser evil is having a yard that looks like it needs to be mowed for three weeks, instead of wasting those seeds. Just wait it out. Once everything has germinated and has a good stand then you can manicure it to perfection. I think you'll benefit more in the end by just waiting.

MMQB - Ok to call me out. I confess to not being brave enough to scalp the little bit of lawn that I have. And I will follow everyones advice to resist the urge to mow it. Thanks.

Why not just raise the setting from the usual and cut the lawn that needs to be cut.
Surely the growing seeds is not that long that the mower will have any effect on.
In the meantime though, if you wait too long, any mowing will cause such clippings to cover over the seed--and in this damp weather, rotting clippings could have a damaging affect on any just erupting seedling.
You cant very well leave it too long without resorting to using a bag to remove the clippings...or heavens, raking.

I'd say as long as the new grass is not being cut, mow the lawn as usual and trust the weather will slow things down somewhat.

"Why not just raise the setting from the usual and cut the lawn that needs to be cut. "

Because that will suck up any ungerminated seeds and can also pull grass up that has germinated but not yet set roots. Lawn mowers exert a vacuum effect to pull the grass up straight before cutting.

If it gets long enough to require bagging, so be it. That's better than killing the new grass.