The letter will be returned to you for lack of postage, unfortunately.
If you sent it Saturday, then it could have gotten through with the 39 cents, but it was dropped Sunday which means the PO will process it only on Monday and that means it has to have the 41 cents stamps. Sorry
But I looked at some articles, and you may get a little leeway. From Denver Post http://www.denverpost.com/nationworld/ci_5888372
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Because most post offices are closed on Sunday, officials say items dropped in a box that will not be collected until Monday should have the higher postage on them.
On the other hand, when rates change the agency usually allows a little leeway, and it does not plan a rash of returns for insufficient postage.
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To be sure call USPS at 1-800-275-8777.