If you're planning to visit Singapore, it's important that you learn about the local public holidays ahead of time. That way, you'll know if there will be any special celebrations, or if the monuments you're interested in will be closed.
New Year. 1st January
Chinese New Year: between January and February (depending on the phase of the moon)
Good Friday: varies
Labour Day: 1st May
Vesak: May (full moon). Celebration of the Buddha. The most sacred day in the Buddhist calendar.
National Day: 9th August. Celebration of the proclamation of the independence of Singapore on the 9th of August 1965.
Hari Raya Puasa: between October and November (date varies). The Islamic celebration of the end of Ramadan.
Diwali: October (date varies). Celebration of the Hindu new year.
Christmas: 25th December
Hari Raya Haji: between December and January (varies). Celebration of the end of the annual pilgrimage to Mecca