Here’s the release schedule and planned content, according to Xseed:
Part 1 — April 2021
Animal costumes for players and marriage candidates Olive Town Mystery Files scenario
Windswept Falls expansion, featuring a new area with four returning characters from previous SoS games. Two of these are marriage candidates.
Part 3 – June 2021
School Uniforms costume set for the player and marriage candidates Terracotta Oasis expansion, introducing an oasis area with four more returning SoS characters. Two of these are marriage candidates.
Part 4 – July 2021
Yukata costumes for the player and marriage candidates The Legendary Sprite Dance scenario
Part 5 – August 2021
Twilight Isle expansion, a new, twilit island area home to four Trio of Towns characters, two of whom are marriage candidates.
As if all the existing customization options and DLC costumes weren’t enough, Pioneers of Olive Town expansion pass owners get a bonus DLC set with costumes from Friends of Mineral Town and Trio of Towns characters once Olive Town releases on March 23 for Nintendo Switch.
