USPS First Day of Sale Rules
Since January 2007, all new stamps and stationery items go on sale nationwide on the official first day of issue date. So reads the USPS Domestic Mail Manual, but local Post Office staff often do not know this policy and will refuse to sell stamps until the following day.
The policy change was stated clearly in the Postal Bulletin of September 11, 2008:
For your convenience, use this link to download a PDF of the relevant page, with highlighting, from this site.