If I don't place all the Seasonal rewards that I get from the 5 Star boxes before the end of the month..

If you opened 5-star pink boxes and received the Marvelous prizes but did not place them on the island, at the end of the month, they will go directly into your storage boat, with red ribbon still intact. You will not lose them, and they will not change to next month's prizes.

However, if you save the 5-star pink boxes, leave them unopened until next month, then any Marvelous prizes you receive will have next month's prizes, since the prize is determined at the time you open the box and not the time you earn the box.