UPDATE: Nodding Heads changed their original post now stating "...from the upcoming Wednesday (and for the duration of the Steam Game Festival) you will have the chance to play an all-new demo of Raji: An Ancient Epic..." So we expect the festival to start March 18th. There still hasn't been an official statement so time will tell.
A post on Twitter from developer Nodding Heads Games stating that their isometric game, Raji: An Ancient Epic, will be featuring a playable demo during the Steam Game Festival may have inadvertently revealed the dates for the upcoming festival.
Their steam announcement also reads, "From to upcoming Monday to Friday (for the duration of the Steam Game Festival) you will have the chance to play an all-new demo of Raji: An Ancient Epic. This demo will let you play through the first level of the game - the ruins."
Nodding Head Games is based out of Pune, India. After converting the date stamp of the post into India's timezone it goes to show that it was posted on Monday March 16th. Based on the post stating "Upcoming Monday to Friday" we're going to surmise that the Steam Game Festival will be featured next week between the dates of March 23rd to the 27th. Of course we'll update this post if come morning the event has begun.
A Steam Game Festival was recently featured on December 12th and 13th 2019 where a number of demo's where available to the public for only 48 hours. Its a bit suspect that another festival would come 3 months after, but with the current state of quarantine that the world is experiencing it sure would be a welcome event. I was unable to find additional information on Steam's website which displays the previous event dates. Flexing some google-fu, I was unable to find any additional information sans Reddit talking about the possibility of the event.