To claim an itemized deduction for a donation of more than $250, generally you need a contemporaneous written acknowledgment from the charity. “Contemporaneous” means the earlier of
1) the date you file your income tax return, or
2) the extended due date of your return.
If you made a donation in 2018 but haven’t received substantiation and you’d like to deduct it, consider requesting a written acknowledgment from the charity and waiting to file your 2018 return until you receive it. Additional rules apply to certain types of donations.