Learn Dutch: Ik voel me nostalgisch over de week dat deze mijlpaal was gevierd. - I feel nostalgic about the week that this milestone had been celebrated.

Normally, the relative pronoun for a de-word is die. However, when referring to a period of time to mean 'when' or 'in which', Dutch speakers frequently use dat. It acts as a smoother alternative to waarin (in which) for time expressions.

Why is the passive voice built with was gevierd instead of werd gevierd?

Werd gevierd is the simple past passive ('was celebrated' or 'was being celebrated'). Here, we need the past perfect passive ('had been celebrated') because the celebration was already entirely completed before the feeling of nostalgia. The Dutch past perfect passive is formed with the auxiliary verb was or waren plus the past participle.

Can I drop me and just say Ik voel nostalgisch?

No, in Dutch, expressing how you feel emotionally or physically requires the reflexive verb zich voelen. If you just use voelen without the reflexive pronoun me, it implies physically touching or sensing an external object, like Ik voel de wind (I feel the wind).

Is over the standard preposition to use with nostalgisch?