why CE instead of CES, PASSÉES instead of PASSÉE in this example?

Cesar,

 ‘ces’ is a demonstrative adjective and cannot be used as a pronoun as required in this sentence. ‘ Ce sont …’ is the plural demonstrative pronoun-verb form required.

Ce, cet, cette, ces + [durée] -là/-ci = that/those or this/these + [duration] (French Demonstrative Adjectives)

C'est, ce sont = this is, these are (French Demonstrative Pronouns) 

In the relative clause, “ qu'elle ait passées “,   ‘que’ is the direct object of the compound avoir- conjugated verb ‘ ait passées ‘, precedes the verb and is the pronoun representing ‘ les vacances ‘. As the direct object precedes the verb, agreement of the past participle with the direct object is required, hence plural feminine form. Note that although the lesson refers to  the most commonly used compound tense, passé composé, the agreement rule holds for all compound tenses with the auxiliary ‘avoir’.

( If agreement was not required, it would be ‘ ait passé ‘, as it would be for masculine singular with agreement ).

 

Que = Whom/which/that (French Relative Pronouns)

 Special cases when the past participle agrees (in number & gender) when used with 'avoir' in the compound past in French (Le Passé Composé)