"His vacation is in July."
Translation:Ses vacances sont en juillet.
"Vacation" meaning a few days off is "les vacances", a feminine plural noun in French.
"Ses" is the possessive adjective you have to use whenever the object owned by a third party (il, elle, on) is plural: ses vacances.
When the possession is masculine, you use "son".
When the possession is feminine and starts with a vowel sound, you use "son" again, to avoid the vowel sound conflict.
When the possession is feminine and starts with a consonant, you use "sa"