Web giant Google announced back in October that it would be revamping the user interface for the web version of its calendar app, and that revamp will begin making its way out to all users very soon. When the rework first became available, users had to actually opt-in to it, and it was only available to a limited crowd. As of January 8, any users on G Suite's rapid release schedule will begin to see the new UI, unless they manually chose to opt out of it at an earlier time. The UI will reach users that previously opted out on February 5, and the rollout will be complete as of February 28, with all users migrated to the new UI and no available option to use the old one.
While users on G Suite's rapid release track will start seeing the new UI as early as January 8, G Suite users on the scheduled release track will have to wait until January 15. The specific date that automatic upgrading will begin for normal users who did not previously opt out of the new UI was not specifically mentioned, so it will presumably be some time before the February 28 deadline. Users who wish to can manually upgrade to the new UI at any time, regardless of previous opt-out status.
The new UI update for Google Calendar brings overtones of the mobile interface to the desktop version. While the current interface does hold itself to Material Design conventions, the mobile version, and by extension the new UI as well, embraces things like vertical scrolling, horizontal columns, and resizing of UI elements that depends on a user's screen size and resolution. This interface lays out information vertically in a block format, presenting users with their events and reminders set along vertical strips representing individual days. This new interface boasts all of the same features as the old one, but is geared more toward supporting a wide range of viewing formats. This may be another step in Google's quest to converge its various software landscapes, with Chrome, Chrome OS, and Android all beginning to have more and more features and user interface conventions in common across apps in the last few months.