Singapore’s new flag was officially unveiled December 3, 1959 to coincide with its self governing status.

State flag of Singapore
National Day 1960

The national flag of Singapore comprises two equal horizontal sections – an upper red section and a lower white section. A white crescent moon occupies the upper left red section, with five white stars arranged in a circle beside it.⁷

Each colour and symbol on the flag has a distinctive meaning and significance attached to it. The colour red symbolises universal brotherhood and the equality of man, while white represents pervading and everlasting purity and virtue.⁸ Together, the two colours signify Singapore’s goal to achieve brotherhood and equality through purity and virtue.⁹ The crescent moon represents a rising young nation, while the five stars depict Singapore’s ideals of democracy, peace, progress, justice and equality.
Singapore’s national flag

The Singapore National Library has an interesting section on how the design was created in 2 months Singapore’s national flag.