Every day at 05:00 (CET), the server goes through a server save: a short, automatic maintenance cycle that keeps the whole world consistent, not just a backup running quietly in the background.
The most visible part: the world is restored from the original map. Items that were dropped on the ground during the day -- by players, by creatures dying, by anything -- disappear, and the original map items are put back exactly where they belong. Nothing in your inventory or your depot is touched by this; it only affects what is lying on the ground in the world itself.
It is also the only moment any house-related change actually takes effect: auctions that have already closed are resolved and handed to their new owner (assuming the winning bidder had the gold ready, see our houses article), monthly rent is collected or a warning letter is issued, overdue tenants are evicted, and any voluntary house transfer scheduled by its owner is carried out. None of that happens the instant you click a button on the site -- it all waits for this one moment each day.
The downtime itself is brief, usually just a few minutes. If you are online at 05:00 CET, expect to be disconnected momentarily while the world restarts -- logging out safely beforehand, if you can, avoids losing your current in-game position.