I was on page four on the second week and was not finished, but now the page is gone so I don’t know how I can finish getting the last item.
The event finished yesterday. That’s why you can’t see it anymore. Thus far, events have only lasted 7 days.
I'm in a similar boat. I was under the impression that once you unlocked the last page, you could complete the last tasks at your leisure. Didn't realize the tasks would disappear if you didn't complete them once the event ended. I thought it would be similar to pokemon go special research tasks which don't disappear until the next special research task is available. I'm not sure why I thought this, but clearly I was mistaken, too. Now I have a hole in my event registry. So that's fun.
Edit: ignore this, misread question...
The registry pages for past events are still there. Swipe left or use the icons at the top of the special event registry page.
Even though the previous event is over you can usually still collect limited fragments from fortress challenges and old brilliant portmanteaus.