U.S. Government Securities Business Day (as defined in 2006 ISDA Definitions Section 1.11 and 2000 ISDA Definitions Section 1.11)
Indeed, Section 1.11 of the 2006 ISDA Definitions defines this as follows:
Section 1.11 U.S. Government Securities Business Day. "U.S. Government Securities Business Day" means any day except Saturday, Sunday or a day on which the Securities Industry and Financial Markets Association recommends that the fixed income departments of its members be closed for the entire day for purposes of trading U.S. government securities.
That USGS is the fixing calendar makes sense. Indeed this is recognised as the publication schedule by the New York Fed, see SOFR footnote b, which states:
The Treasury repo reference rates will be published each business day that is not recognized as a holiday by the SIFMA calendar for secondary market trading of U.S. government securities. The Treasury repo reference rates, reflecting activity for the business day preceding a holiday will be published on the subsequent business day. Please note that on days in which trading in U.S. government securities is subject to an early close, the reference rates administered by the Desk will still be published. In the event that market participants recognize a previously unscheduled holiday, the New York Fed will publically communicate its approach to publishing reference rates it administers, with the goal of aligning as closely as possible to the approach used for scheduled holidays.
However, a key difference between holiday calendars in the USNY and USGS business centers is the treatment of Good Friday, and when SIFMA recommends that it is observed as a partial trading day, with an early close.
When is Good Friday a holiday in the holiday calendar for the USGS business center?