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 in January 2021 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

In Japan, the top 100 players of each team and 100 random others will receive a special prize: a Super Mario Bros. 35th anniversary medal.
 * Collectible medals

In Europe, It was revealed on 2020-11-04 that 200 medals will go to the top 100 players on each team.

Trivia

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