The origin of this tradition is not really clear. Some say it started in the Middle Ages: on this Monday at the beginning of the new year, the new officials were sworn in, and this was celebrated with food and drink. Others say it started in the 18th century, when the guilds were celebrating the new year. Since it was not a work day, it was called a “lost” day.
“Lost Monday” doesn't fall on a specific date; it is the first Monday after January 6, the feast of the Epiphany, as long as a Sunday has intervened.
Mark January 11, 2021 on your calendar now. That's when it will be “Lost Monday” again!