Per IRS Publication 502, Medical and Dental Expenses, page 2:

What Expenses Can You Include This Year?

You can include only the medical and dental expenses you paid this year, but generally not payments for medical or dental care you will receive in a future year. (But see Decedent under Whose Medical Expenses Can You Include, later, for an exception.) This is not the rule for determining whether an expense can be reimbursed by a flexible spending arrangement (FSA). If you pay medical expenses by check, the day you mail or deliver the check is generally the date of payment. If you use a “pay-by-phone” or “online” account to pay your medical expenses, the date reported on the statement of the financial institution showing when payment was made is the date of payment. If you use a credit card, include medical expenses you charge to your credit card in the year the charge is made, not when you actually pay the amount charged.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.