Updates, repairs, etc. done irregularly buy Youtube
. In order to perform such maintenance's the site must be all but totally shut down, meaning you can watch videos on the site but you cannot comment on a video, favorite a video, send a video, or rate a video. You also cannot send anyone a message, post a comment on someone's channel, send a friend invite, or subscribe to someone.
In the past Youtube posted messages warning users about maintenance that evening, but after major page redesigns the only message that appeared was the one informing you maintenance was currently in progress, but now they've eliminated that too, meaning the only way you know is when you can't do anything but watch vids.