I printed a wrapper with postage already pre-paid and I put the date being mailed as the 5th. If I do not actually mail it the 5th, will it come back to me? Will it make a difference if I put it in a mailbox on the 6th?

You can mail earlier than the date, but not later.  The only way to get around it is if you drop the package in a USPS blue mailbox before the first pick-up on the 6th.  For all they know, you could have dropped it in the evening of the 5th.  If that doesn't work - you have to take it to the post office and they'll fix the date for you.  HTH!

ok, thanks!

if some reason you dont make till the 6th you could try to to post office and see  if they would give youa zero label

for the correct date

I've printed labels in the evening before and forgotten to change the date. I put them in the box the next morning and they were fine.  You can take them into the PO  and ask them to adjust the date. 

One night I got to the PO too late after it was closed and had to go back the next morning to ask the clerk to put a 00 label; she threw a hissy fit.  The clerk told me I should have taken the letter to the PO by the airport which closes at midnight.  Right, I wouldn't be able to get out there without a car and it would be $35 for a cab.  The letter wasn't urgent and I thought I was being honest by bringing it the next day and asking for help  Since then, I've dropped mail that was 1 date after the postage stamped in the blue box and haven't had a problem. 

i always forget to change the date. here and when i make labels with paypal. idk why no one thinks to post date the things when u make a label at 10pm.

i've mailed them a day later then what was printed on there. so far nothing has come back. i bet they will leave it alone as long as its just a day or two.