For personal taxes the timing rules differ for tax-deductible items, such as charitable donations. So long as the checks are in the mail by year-end, you are able to lock in a 2016 write-off.
For deductible charges made with a bank credit card, you can claim the write-off in the year that you charged the expense, even if you pay the bill in the next year.
Different rules apply for deductible charges you make with a retail store credit card. You can take the write-off only for the tax year in which you actually pay the bill.