If you’re shipping holiday cards and gifts for Hanukkah (Dec. 25–Jan. 2), Christmas (Dec. 25), Kwanzaa (Dec. 26–Jan. 1), or other holiday traditions, the US Postal Service says the earlier you send, the better. 

The recommended send-by dates for expected delivery on or before December 25 are:
USPS Ground Advantage Service: Dec. 18
First-Class Mail: Dec. 18
Priority Mail: Dec. 19
Priority Express Mail Service: Dec. 21

Source: usps.com