In many years, December 21 is the Winter Solstice; the video might feature specific lighting or astronomical events related to the shortest day of the year.
This naming format is the default for the iOS Control Center screen recording tool.
Recordings of school plays, gift exchanges, or winter activities.
Right-click the file and select Get Info . Look under "More Info" for dimensions, codecs (usually H.264 or HEVC), and duration.