From IRS Publication 557

A donor can deduct a charitable contribution of $250 or more only if the donor has a written acknowledgement from the charitable organization.  The donor must get the acknowledgement by the earlier of:

1) The date the donor files the original re-turn for the year the contribution is made, or

2) The due date, including extensions, for filing the return. The donor is responsible for requesting and obtaining the written acknowledgement from the donee.

Quid pro quo contribution. If the donee provides goods or services to the donor in exchange for the contribution (a quid pro quo contribution), the acknowledgement must include a good faith estimate of the value of the goods or services. See Disclosure of Quid Pro Quo Contributions, earlier.

Form of acknowledgement. Although there is no prescribed format for the written acknowledgement, it must provide enough in-formation to substantiate the amount of the contribution. For more information, get IRS Publication 1771, Charitable Contributions – Substantiation and Disclosure Requirements.