How are UN official holidays decided?

The UN Staff Rules state that "The Secretary-General shall establish a normal working week and shall establish official holidays for each duty station". (see Staff Rules and Staff Regulations of the United Nations, regulation 1.3 (b))

Towards the end of each year, duty stations issue a list of the holidays for the upcoming year. In New York and Geneva these are issued as information circulars (ST/IC).

Over the years, the General Assembly has adopted resolutions and decisions requesting the Secretary-General to increase the number of holidays and add new days.

Points to Note:

• In 1946 and 1947, Information Circulars were issued for each Official Holiday under the symbol: ST/IC/--
• In 1948 one document was issued under the symbol ST/SGB/68 and a separate Information Circular under the symbol ST/IC/491. 
• In 1949 and 1950, there are no documents found on Official Holidays.  (Official Holidays are addressed in staff rules 101.2 and 201.3)
• From 1951-1976, Official Holidays are issued under the symbol:  ST/AFS/SER.A/---
• From 1977-Current Official Holidays are issued under the symbol: ST/IC/YYYY/---
• In 1982, the number of Holidays is increased from eight to nine in ST/AI/283.  In the same Administrative issuance, staff rules 101.2 and 201.3 are amended to remove National Holidays as Official Holidays and increase the number of Official Holidays to nine including “…One day during the Christmas Season, designated each year by the Secretary General  ...”
• ST/IC/1996/68 is the last one that grants “…One day during the Christmas Season, designated each year by the Secretary General  ...” as a UN official Holiday.
• In 1997,  two more holidays are added: Id Al-Fitr and Id Al-Adha, increasing the Official Holidays to ten days (see General Assembly Resolution 51/211, decision 52/468, ST/IC/1997/21 and ST/IC/1998/30).
• In 2015, one of the ten days of Official holidays was set as "floating". Staff members are given the option of observing any one of the following holidays in that day: Orthodox Christmas, Lunar New Year, Presidents’ Day, Orthodox Good Friday, Norwruz, Yom Kippur, Day of Vesak, Diwali, Gurpurab (see General Assembly resolution 69/250, ST/IC/2015/26, A/RES/72/19 and A/RES/78/245).
• In 2021, the last ST/IC for official holidays for New York (HQ) for the year 2022 (ST/IC/2021/14) was issued. The information circular noted that "Information on the official holidays for 2023 and subsequent years will be shared through other communication channels and not through an information circular." 
• Information circulars for the official holidays for the UN Office in Geneva continued to be issued to date.