More specifically, how do I tell which items in the quest shops will be expiring at the end of summer? If there's a list on Subetalodge, I can't find it.

Edit to add: sorry for all the questions I've been asking recently.

Deadlines: Retirement Reminders lists all retiring items and ending events :) Seasonal changing items are the most expensive and last listed items in Quest Shop.

thanks for the extremely helpful answer. :) Going to have to max out quests until retirement so I can try to nab all of those summer items.