Which is correct?
"In the summer...." or "In summer...."

Both are fine, and without context, it is difficult to advise which to use.

Generally, 'in the summer' has the connotation of one summer (e.g. next summer); whereas, 'in summer', holds the idea of every summer.

It depends on text.If you mean summer in general then, in summer is correct, if you mean a specific summer so you should use The.