The noun (Substantiv/Nomen) Sommerferien is feminine in German. Therefore, the correct definite article in the nominative singular is die. So, you say: die Sommerferien.
summer holidays
This word is composed and its gender is always determined by the root word.
Die Ferien im Sommer, die längste Schulferienzeit in Deutschland.
The summer holidays, the longest school holiday period in Germany.
Wir fahren in den Sommerferien ans Meer.
We are going to the sea during the summer holidays.