Valve held the first Steam Game Festival back in December 2019 along with Geoff Keighley and the two parties are organising another one for the spring of 2020, featuring over 40 games that couldn't showcase themselves at conventions.
Steam Game Festival in late 2019 ran for two days, showcasing games alongside The Game Awards. The Spring 2020 edition will kick it up a notch, consisting of more than 40 games which is a much larger selection of titles with featured demos than the one previously available.
Furthermore, the surprise Steam Game Festival will run for five days, starting on 18 March 2020 and wrapping up on 23 March 2020. Both the start and end times are at 10:00 AM PDT / 6:00 PM CET. When the event arrives, it will be visible on the dedicated landing page .
According to Geoff Keighley's tweet that revealed the event, the showcase will focus on the titles that were denied demoing on events like The Mix, Day of the Devs and Indie Megabooth due to the cancellation of GDC and other conventions following the health concerns prompted by the Coronavirus outbreak.
While this definitely is a great opportunity for indie devs to showcase their work, it is a bit vague whether the Spring event was made solely for that reason. In a follow-up tweet, Keighley noted that more editions of the Festival are planned in 2020, mentioning the "content you can play at home".
On top of that, there are apparently plans to make the Festival available on more platforms in the future.