Questions & Answers about Vi vil fejre min fødselsdag i haven.
Both are grammatically possible, but the nuance is different.
Vi vil fejre min fødselsdag i haven.
Emphasises intention or decision: We want to / are going to celebrate my birthday in the garden.Vi fejrer min fødselsdag i haven.
In the right context (especially with a time expression), this can express the future:
Vi fejrer min fødselsdag i haven på lørdag. – We’re celebrating my birthday in the garden on Saturday.
Without any time expression, Vi fejrer min fødselsdag i haven. can more easily be understood as something habitual or generally true (for example, a tradition), whereas Vi vil fejre… sounds more like a specific plan or intention.
Both can talk about the future, but they feel different:
Vi vil fejre min fødselsdag i haven.
Focus on will/intent/want:
We want to / intend to celebrate my birthday in the garden.Vi skal fejre min fødselsdag i haven.
Focus on plan/obligation/something arranged:
We’re going to / supposed to / are scheduled to celebrate my birthday in the garden.
So vil is more about desire or intention, skal more about an arrangement or necessity. In many everyday contexts both are possible, with only a subtle difference in feeling.