To those worried about having to wait even longer to get Saki once Trading Post rotation starts:

In the year I've been playing at DMM, she's never rotated off while the platinum and gold units rotate monthly.

As for Memento's event, the original is a star trial so might as well go for perfect if you can. She's not a bad unit and her tokens don't count against 3 start requirements, so they've great for things like delaying bosses