Remove all predefined holidays.
(with-eval-after-load 'calendar (setq holiday-general-holidays nil holiday-solar-holidays nil holiday-bahai-holidays nil holiday-christian-holidays nil holiday-hebrew-holidays nil holiday-islamic-holidays nil holiday-oriental-holidays nil))
Define the public holidays in Slovakia.
(with-eval-after-load 'calendar (setq holiday-local-holidays '((holiday-fixed 01 01 "Establishment of the Slovak Republic") (holiday-fixed 01 06 "Epiphany") (holiday-fixed 05 01 "International Workers' Day") (holiday-fixed 05 08 "Victory over fascism") (holiday-fixed 07 05 "Saints Cyril and Methodius") (holiday-fixed 08 29 "Slovak National Uprising") (holiday-fixed 09 01 "Constitution Day, 1992") (holiday-fixed 09 15 "Day of Our Lady of the Seven Sorrows") (holiday-fixed 11 01 "All Saints' Day") (holiday-fixed 11 17 "Struggle for Freedom and Democracy") (holiday-fixed 12 24 "Christmas Eve") (holiday-fixed 12 25 "Christmas Day") (holiday-fixed 12 26 "St. Stephen's Day") (holiday-easter-etc -2 "Good Friday") (holiday-easter-etc +1 "Easter Monday"))))