From the USA.gov Blog:
How to Put Your Mail On Hold:
The U.S. Postal Service can hold your mail if you plan to be away for three or more days (up to 30 days).
When you fill out a request for Hold Mail Service, you’ll need to enter the start and end dates and indicate whether you prefer to pick up your accumulated mail or have it delivered.
If you’ll be away for more than 30 days, the U.S. Postal Service offers mail forwarding options.