In summer or In the summer?
Exemple : ‘’I don't wear jeans in summer when it is hot.’’
The difference between "in summer" and "in the summer" is negligable and it can be argued that either can be used in all situations. It is, however, argued that "in summer" should be used to refer to something which happens every summer.
In the summer
Exemple : ‘’I visited Venice in the summer holidays.’’
"In the summer" refers to a particular summer rather than to summer in general.