Yes, the United States Postal Service (USPS) is mandated to deliver mail six days a week. This requirement stems from federal law, specifically from funding legislation passed by Congress. Since 1983, various appropriations bills have included provisions requiring the USPS to provide six-day delivery service. For example, the Consolidated Appropriations Act of 2021 (and similar prior acts) explicitly states that the USPS must maintain six-day delivery as part of its universal service obligation, unless explicitly altered by Congress.

This mandate reflects the USPS's role as a public service ensuring consistent mail access across the country, though there have been debates and proposals over the years to reduce it to five days to cut costs—none of which have been enacted as of March 15, 2025. So, barring any new legislation I’m not aware of up to this point, the six-day delivery remains in place.