"In summer students have holidays."

It's actually the same construction, but with є omited.

Є is only used when 'existing' or 'having' is a part of the new information told in the sentence:

• Влітку у студентів є канікули tells us two pieces of new information: 'In summer, students have something (some time period). That something is holidays.' This sentence can be an answer to the question 'Do students have vacations in summer?' or 'Do students have vacation?'. The fact that they indeed have something it is the new information.
• Влітку у студентів канікули tells us one piece of new information: 'Something that students have in summer is holidays.' This sentence can be an answer to a question 'What do students have in summer?'. The fact that they have something in summer is assumed.

N.B. The description above is about the standard Ukrainian as spoken in Ukraine. Some dialects, like the one spoken by Zonia435615, use є differently. I think she would use «є» in both those sentences. This is characteristic of some Western Ukrainian dialects, while the standard Ukrainian is based on the central Ukrainian dialects.