Super Mushroom vs. Super Star

Super Mushroom (Super size) vs. Super Star (Invincibility) is an upcoming Splatfest event in Splatoon 2. It will be held globally from 2021-01-15 to 2021-01-17 to celebrate the 35th anniversary of the Super Mario Bros. series. It was announced during the Super Mario Bros. 35th Anniversary Direct on 2020-09-03.

Details
The Splatfest theme is about two power-ups from the Super Mario Bros. series: the Super Mushroom, which grants super size, and the Super Star, which grants invincibility.

Splatfest T-shirts based on the artwork for the event were made available to purchase at the My Nintendo Store soon after the announcement. They are also available on Nintendo's online stores for South Korea and Hong Kong. Also, Splatfest keychains can be bought for 100 Platinum Points in North America, Europe, and Japan.
 * Merchandise

To celebrate the Super Mario Bros. 35th Anniversary, special medals are available for participants in Japan and Europe. 200 medals will be delivered to the top 100 players of each team and in Japan only 100 random others will receive the medals as well. On 2020-12-09, Nintendo of Japan revealed further information about the collectible medals within Japan:
 * Collectible medals

Further lottery for 100 people:

Whether you participate in regular or challenge, you will be eligible for the lottery. Precautions for participation:
 * Double chance for those who participated in the event even if they did not rank high! The same premium medal will be presented to 100 of all participants by lottery.
 * It is a condition that you have participated in at least one festival match within the period.
 * Please participate in the festival with the aim of getting a medal!

Please note the following regarding this event. Please check in advance before participating.
 * Please note that if you change the "Country / Region" registered in your Nintendo Account after the event, you may not be eligible to receive medals.
 * Those who perform actions that Nintendo deems inappropriate, such as fraudulent results or intentional disconnection of communications, may not be eligible for the gift.
 * If there are multiple players with the same festival power, the player who reaches the festival power value first will be ranked higher.
 * A maximum of one prize can be presented for each Nintendo Switch. If you win in the top 100 with multiple Nintendo Accounts from one main unit, the account with the highest festival power will be eligible for the premium medal.
 * The result of the event will be notified by e-mail only to those who are eligible for the gift. Please note that we will not answer individual inquiries such as winning the lottery or ranking of individuals.
 * To participate in the festival, you need to subscribe to the Nintendo Switch software "Splatoon 2" and "Nintendo Switch Online (paid)".
 * There are also notes on shipping prizes as well.
 * Nintendo will contact the recipients of the gift to the email address registered in their Nintendo Account by February 28, 2021. Please check again if your email address is valid. If you do not receive the email, your winning will be invalid.
 * If a player using a child account is eligible for the gift, an email will be sent to the registered email address of the "Family" administrator of that account.
 * Those who are eligible for the gift will be required to complete the free shipping procedure for this prize at the My Nintendo Store. At that time, you need to enter the shipping information such as your address.
 * Prizes can only be shipped within Japan.

Trivia

 * This is the first Splatfest with an original theme since the event that was officially dubbed as the final one: Chaos vs. Order.