On August 28th, the Wonderful Holidays suit will be available through sign-in rewards!
For each day that you log in, you will be able to claim a part of the suit for free. Click on the Events tab, and then the Wonderful Holidays banner, where you will be able to claim one item per day.
Also note that between August 28th and September 8th, you will be able to collect the Berry Powder Shaken suit and Wonderful Holidays suit at the same time (if you have not already completed Berry Powder Shaken). After September 8th, you must complete the Wonderful Holidays suit before you can begin collecting the Berry Powder Shaken suit.