Religious holidays run according to the Muslim lunar calendar. Because this is eleven days shorter than the Gregorian calendar, holidays move forward 11 days each year, and exact days should be checked with the tourist office. Most holidays/festivals won't affect visitors, however it is worth noting:
Ramadan Business hours may be shorter and restaurants shut during daylight hours.
Seker Bayrami Sugar Festival comes at the end of Ramadan, it's a three day national holiday and everything shuts.
Kurban Bayrami The most important festival of the calendar and must be planned for as everything officially shuts for four days.