When overseeding a new lawn you should wait to overseed the new lawn at least 45 days before your average first fall frost.
If you're in a southern area then you can overseed the lawn that is thinning in late spring as the warm season grasses enter active growth.
You can overseed the new lawn a few weeks to about a month after planting the new grass seed.
I usually overseed a few weeks after planting the new grass seed to allow the grass to grow properly and I've never had any problems.
I've always had my new lawn turn out really nice as a result.