You have asked two questions, here, Ann.
1. Is there a difference between vacation and holiday?
Yes, and the difference is mainly a regional one. As the American members have explained, 'holiday' is only used in the US to refer to public holidays of a day or so, in particular religious or commemorative occasions, such as Christmas or Thanksgiving. Longer periods, such as summer breaks from school, and private leisure trips, are called vacations.
Outside of North America, the word 'vacation' is rarely used. 'Holiday' refers to all of the above.
2. Your sentence should read "I can't wait for the summer holidays." You should use the plural form, because this refers to the long break between summer and autumn school terms.