The Rainbow Spire leaves the game on the 10th, so if you haven't crafted it and don't have the pieces you have the option of getting them for 300 Gold. This is a decoration only.
If you didn't complete the goals for Rainbow Sentinel, it's expired so there would be a goal in your goal book which would have popped up to let you know that it was expired. You have the option to buy the Dragon for 1700 Gold or say No Thanks. You'll only see this once and once you make your choice the goal will disappear from the log. If you choose to buy one, it'll go into Storage and you can hatch it from there.