In the USA, total medical expenses above a threshold may be deducted from taxes. If a medical expense occurs in December and the bill is paid in January, for tax purposes, does this expense occur in the December year or the next (January) year?
The expense occurs when you pay it, so the expense would be deductable for the year of that January. (Source: working with a tax accountant last year when I was in a similar situation. Disclaimer: I am not an accountant.)
Additional source, IRS pub 502, at the end of page 2:
You can include only the medical and dental expenses you paid this year, regardless of when the services were provided.