in session

in session

[of a court, congress, or other organization] operating or functioning. Use of cameras is forbidden while the meeting is in session. The spectators must remain quiet while court is in session.
See also: session
References in classic literature ?
The ordinary power of appointment is confined to the President and Senate JOINTLY, and can therefore only be exercised during the session of the Senate; but as it would have been improper to oblige this body to be continually in session for the appointment of officers and as vacancies might happen IN THEIR RECESS, which it might be necessary for the public service to fill without delay, the succeeding clause is evidently intended to authorize the President, SINGLY, to make temporary appointments "during the recess of the Senate, by granting commissions which shall expire at the end of their next session.
A DISTINGUISHED Naturalist was travelling in Australia, when he saw a Kangaroo in session and flung a stone at it.