See IRS Pub 526: https://www.irs.gov/pub/irs-pdf/p526.pdf
It must be "delivered" in 2020 and is considered delivered at the time of its unconditional delivery. To be a contribution that is reportable on your 2020 tax return, a mailed check would generally have to be postmarked no later than December 31, 2020 (otherwise you would not be able to substantiate that you put it into the mail by that date). If you hand-deliver it, it must be delivered by December 31, 2020. Writing a check on December 31, 2020 but not mailing it until January 2021 would make it a 2021 charitable contribution.
