Here are the cutoff dates for when you should send mail out so that it arrives by December 25th:
USPS Retail Ground Service: Dec. 15
First-Class Mail Service: Dec. 18
Priority Mail Service: Dec. 19
Priority Mail Express Service: Dec. 23
"If sending something to Alaska, Hawaii or overseas, it’s not too late, but you’ll have to act faster. Click here for when best to mail Christmas items to those parts of the world." -(KKTV 11 News)