Holiday is a common term used by Chinese. However, summer is a season and is not technically a holiday. It is more often referred in the USA as a "vacation" or a "break". Both refer to a period of rest from one's normal routine (academics in this case). I would reserve the word holiday for actual holidays. For example, Christmas, New Years, etc. In the USA, another popoular "break"/"vacation" is "spring break" or "winter break". These are periods of rest between school terms. In winter break, there are two holidays - Christmas and New Years. For spring break, many young people vacation and it is near the Easter holiday.