How it is built, said plainly
Anniversary is not one of the dedicated event designs yet; on the app it appears among the occasions still to come. What you get today is the same thing the platform puts behind any gathering, and for an anniversary it is enough: a shareable event page, replies tracked in one guest list, reminders to the people who have not answered, and a link you can forward without it degrading.
So the useful decisions are content decisions rather than design decisions. Decide what the page has to answer in one read, decide who the host name is, and decide the reply deadline, which for an anniversary is usually pinned to a restaurant booking or a venue minimum.
If the evening involves people travelling, keep that section short and specific: the arrival day, where you have put them up or recommended, and who they call if it goes wrong. Anniversary gatherings mix generations, and the eighty-year-old guest needs different information from the cousin driving an hour.