The list of 2021 Serbian public holidays does clarify what happens when the day in lieu for May Day, occurring on a Saturday in 2021, conflicts with Orthodox Easter Monday.
The 2011 "Law on State Holidays" also lists holidays which are working days (У Републици Србији се празнује и обележава радно), such as Saint Sava’s Day, National Holocaust, WWII Genocide and other Fascist Crimes Victims Remembrance Day, and Saint Vitus’ Day.
The declaration of the Julian calendar (Orthodox) New Year on Monday, January 14, 2013, as an official non-working public holiday in Serbia (2013-01-11), was a one-off event (2015-01-12 and 2013-12-17).
Links and References
Below are links to the news stories referred to in the above "Serbian 2021 Public Holidays Announced" news story, as well as links to subsequents news stories which refer to the present news story.