For Western Christians, especially Roman Catholics, Lutherans, and members of the Anglican Communion, Lent begins with Ash Wednesday. For Catholics in the Eastern Rites, however, Lent has already begun by the time that Ash Wednesday comes around.
Clean Monday is the first day of Great Lent, as Eastern Catholics and Eastern Orthodox refer to the Lenten season. For both Eastern Catholics and Eastern Orthodox, Clean Monday falls on the Monday of the seventh week before Easter Sunday; for Eastern Catholics, that places Clean Monday two days before Western Christians celebrate Ash Wednesday. Therefore, in order to calculate the date of Clean Monday for Eastern Catholics in any given year, you simply take the date of Ash Wednesday in that year and subtract two days. (See When Is Ash Wednesday? for the date of Ash Wednesday in this and future years.)
The date on which Eastern Orthodox celebrate Clean Monday is usually different from that on which Eastern Catholics celebrate it. That's because the date of Clean Monday depends on the date of Easter, and the Eastern Orthodox figure the date of Easter using the Julian calendar. (For more details on the discrepancy between the Western and Eastern dates for Easter, see How Is the Date of Easter Calculated?) In years when Easter falls on the same day for both Western Christians and Eastern Orthodox (such as 2017), Clean Monday falls on the same day as well.