Also note that all tickets are limited to the girls that are out when you get the ticket... there is no reason at all to save them... ever.

That Easter sale specifically said in the information that the pool of girls would not change, you will notice the same thing in all ticket sales.

Same rules apply as gaining event girls with event currency, do it right away with no waiting or you will loose the chance.

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EDIT: Tickets from crossover events may not be the same as in game events when it comes to the pool of girls, but they would be the only exceptions.