Both your sentences are correct. If the reference is not to a specific summer, the article need not be used but it often is. The article is needed if you talk about a specific summer: What are you going to do in the summer?
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AnonymousSo what is the difference between the meaning of my two sentences ?There's no difference.
radovan"I met my boyfriend on holiday in the summer"It's certainly not ungrammatical, but since in the summer may refer to any summer, just like in summer, there's nothing in the sentence to suggest that you met him last summer. In other words, in the summer doesn't necessarily refer to last summer. People may understand it correct