Certain events have irregular dates (like Chinese New Year) but mostly around the month I categorize them in.
January
February
Chinese New Year celebration
Chingay Parade
March
Mosaic Music Festival
April
World Gourmet Summit
May
Biennale
June
Singapore Arts Festival
Great Singapore Sale (GSS)
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July
Great Singapore Sale (GSS)
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Dragon Boat Festival
Singapore Food Festival
August
9 - Singapore National Day Parade
September
F1 Singapore Grand Prix
October
Deepavali celebration
November
Hari Raya Haji
SitexDecember
25 - Christmas celebration
31 - New Year's Eve celebration 
Legend:
Religious celebrations
Should not be missed