As with the new gTLDs .page and .app, the latest top-level domain managed by Google Registry, .day also works only with SSL encryption. Currently, the TLD is in the sunrise phase. It will enter General Availability on February 1, 2022. You can pre-register .day domains for all launch phases (sunrise, EAP, General Availability/Live) now.
Tip: Every domain managed with Schlund Technologies comes with a free Basic SSL certificate!