If the lawn company overseeded yesterday, sounds like it's been done. To mow now, you risk sucking the seeds up with the mower.

Oops- I meant the lawn company aerated yesterday and the grass wasn't as short as I hoped. I had the option of mowing and bagging trimmings or moving and trimmings clog holes, or just overseed. I decided to overseed and will wait 2 weeks to mow

If you haven't done it yet, definitely cut the grass low as possible now before seeding. Bagging clippings would be preferable but even if you don't, at least cut the grass low.

bummer, too late. I already overseeded before mowing. I was reading several places online with similar issue and others talked about how mowing after aeration but before overseeding may cause the holes to fill up etc. I live in Massachusetts, so ground is still warm, perfect temps, and I have irrigation. So should be ok to germinate just existing grass will shade the new seeds to some extent.