If you make the contribution by credit card, it is considered made on the date that you charge it to the card, even if you don't pay the bill until the following year. If you make the contribution by mailing a check, it is considered made on the date that you mail the check. To avoid any disputes in case the IRS questions your contributions, it's a good idea to make the contribution early enough to make sure that the receipt from the charity shows the correct year.
