Remember that you cannot operate while you are in the 2 year grace period. During that time the FCC is "holding" your call sign for you but you don't have a valid license.
If you send in a renewal application before the expiration date, but the expiration date passes and FCC hasn't acted on it yet, you can still operate. However, if you send in a renewal application after the expiration date, you can't operate until the FCC acts on it.
In practice, FCC processes routine renewals so fast that the above almost never is an issue.
You can renew online in a few minutes, too. Easy as can be.
73 de Jim, N2EY