According to a post by Disney, once upon a snowman will be on Disney Plus on October 23rd, 2020.
From the screenshot
It all started with a little magic 
Once Upon a Snowman, an Original Short, is streaming Oct. 23 on #DisneyPlus.
Olaf Surfaces in the Movie, ‘Once Upon A Snowman’ as Disney Plus Announces Its Streaming Date, 23rd October 2020