There are four possible things people are referring to when they refer to the legislative session.
First is the biennial session that begins on the first Tuesday after the first Monday of January in an odd-numbered year and ends no later than the first Monday after the third Saturday in May of the even-numbered year. The 2021-2022 session is a biennial session.
The next definition of a session is the yearly session, which begins on the day the Legislature assembles and ends on the day it adjourns for the year.
Daily sessions, which adjourn each day, are any time the House or Senate meet on the floor of their respective chambers. At such times, both bodies are referred to as "in session."
Finally, a special session is one called by the governor at a time other than a regularly scheduled session. The legislature, however, determines the length and purpose of any such session.
For further information, see the House Research publications Regular Sessions of the Minnesota Legislature and Special Sessions of the Minnesota Legislature.