There is sometimes an unofficial "grace period" of 24 hours or so when the price of a first-class postage stamp is increased. However, any such grace period for the 41-cent stamp expired in 2008, when first-class postage was increased to 42 cents for one ounce.
Stamp prices are traditionally increased on Sundays. Since post offices are closed and most mailboxes are not serviced on Sundays, the U.S. Post Office often grants a grace period for letters posted on the Sunday on which stamp prices go up. By the end of the next day, however, the new postage rates are considered fully in effect.
First-class postage was raised from 41 to 42 cents on May 12, 2008. Any forever stamps that were purchased at the 41-cent price, however, were still valid as first-class postage after the price increase.