You just have to be aware as to when events typically are and the monthly event. Eventually, if you played a lot you would figure that on the 2nd week of any month, the monthly event wants you to spend 20 summon scrolls. The 2nd week of the event cycle (which is november 2-8) tends to have the summons rebate. Waiting for the monthly event refresh for the 2nd week while the rebate is up is what people should be doing.

However, the way the event cycle and when november starts, you only have one day to benefit from the summon rebate while doing to summon require for the monthly event since the event cycle refreshes on novermber 9th. Just like what PraiseLuka said, it doesn't matter whether or not the scroll is permanent or not, you can still use them but for the 7days scrolls you just have to use them within the 7days regardless if a rebate is present or not.