Answer:
The answer is "adjectival clause".
Step-by-step explanation:
"when the moon and the sun will align" is an adjective clause because it is modifying the noun "year", because it has a verb and a subject, and because it is introduced by the relative adverb "when".
Adjective clauses, also known as relative or adjectival clauses, always describe the noun or the pronoun they modify.
Another example, in "I can remember the house where I spent my childhood", the noun "house" is being modified by the adjectival clause "where I spent my childhood", which has subject and verb, and it is introduced by the relative adverb "where".