Cambodia has 26 days of public holidays per year and during these days most schools, local companies and administrative authorities are closed. Most schools and local companies work Monday to Saturday but Saturday is only a half day. The administrative authorities on the other hand are only open Monday to Friday and if any of the Public Holidays occur during the weekend they will have holiday on the Monday after instead. 
The two most important holidays during the year are Khmer New Year in April and Pchum Ben in end of September or beginning of October. These holidays are normally celebrated with family at home and at the local pagoda. The biggest festival is theWater Festival that take place in November, this festival is celebrated on the streets near the river in Phnom Penh.
To have 26 days of public holidays can seem a lot if you compare it to the average of 10 days that the Western countries have, but in Cambodia most local companies do not offer their employees any vacation in excess of the public holidays. Many of the western founded companies and organizations operating in Cambodia will not follow all the public holidays instead they make a mix of the most important Cambodian and Western Holidays and then offer vacation on top of that. For example some western founded companies in Phnom Penh that we have talked to have chosen to have holidays during International New Year (1 January), Khmer New Year (13-15 April), Pchum Ben (14-16 October), Water Festival (27-29 November) and Christmas Day (25 December). 
In excess to the Cambodian Holidays some Cambodians with Chinese origin also celebrate Chinese New Year (23-25 January), Qingming Festival (2-4 April) and Mid-autumn Festival (30 September), and might want to have vacation during these holidays.
Cambodia also have a Muslim minority that celebrate the Islamic holidays, the two biggest holidays for Cambodian Muslims are Eid-Ul-Fitr (End of Ramadan, 19 August) and Eid-Ul-Adha (Festival of sacrifice, 26 October).