It's sometimes called "Morning Message." I've heard it called "Community Circle," or just plain "Announcement Time." It probably goes by other names too, but in nearly every single classroom across the country, there is a version of this time honored American school tradition.
Most teachers will do this in the morning, if not first thing than somewhere close to it. I know some who use this as a closure to the day. I've never seen it done in the middle of the day, but I wouldn't be surprised if it happened.
Morning message, is well, what it is. It's the time to give students an outline of what will happen during that day. Assemblies? Guest speakers? When and where to turn in those fund-raiser forms? Now is the time to get everyone caught up on the news. They do the class song, the class chant, the pledge of allegiance, class sharing, etc. If the school as a school-wide morning announcements time, that's also included. This is why most teachers do morning message in the morning.
Some teachers (including JL, the CT that failed me) will throw in an "energizer," or a short game that the entire class gets involved in. I've grown to not like this idea, and not only because I associate it with negative student teaching experiences. It's just a waste of a very peaceful and calm time, when students are still a little dazed and sleepy. This is the time to take care of business - correct and collect homework, take role. With older students I would sometimes even start with a short quiz of yesterday's lessons. Energizers are better saved for transition times later in the day, when students are getting squirrelly in their seats.