Bungie expects the game and the website to be down for two hours, from 10 a.m. to noon PT. Maintenance is the reason for the downtime, according to Bungie's announcement. The studio added in a follow-up tweet that it will be performing server-side work during the two-hour window, which won't affect gameplay.
It's possible that Bungie is using the maintenance period to prepare Destiny for patch 1.1.1, which will introduce some new features, change parts of the existing game and fix some annoying bugs. Bungie reiterated today that it is still planning to release the patch before the end of February.