When you talk about the thirties, you are referring to numbers between 30 and 39. For example, if you are in your thirties, you are aged between 30 and 39. If the temperature is in the thirties, the temperature is between 30 and 39 degrees.
Mozart clearly enjoyed good health throughout his twenties and early thirties.
三十年代(指 1930 年至 1939 年)
The thirties is the decade between 1930 and 1939.
She became quite a notable director in the thirties and forties.