Conversation so far:
Did Bill Shankly play or manage for Carlisle?
Bill Shankly began his managerial career at the club where his professional playing career had started. Carlisle in the 1948–49 season were struggling.
When did Bill Shankly become manager?
Bill Shankly became manager during the 1948–49 season
How did Bill Shankly do in his first season managing Carlisle?
Bill Shankly's work ethic transformed the team who finished 15th in 1948–49. They improved to ninth in 1949–50.
Did Bill Shankly make any big changes to the Carlisle United team?
Bill Shankly used psychology to motivate his players.
Did Bill Shankly bring in any new players to the Carlisle United team?
One of Bill Shankly's players at Carlisle was Geoff Twentyman, then a promising young centre half.
How long did Bill Shankly manage the Carlisle United team?
In 1950–51 Bill Shankly resigned and accepted an offer from Grimsby Town.

Question: When did that happen?
Self-contained rewrite: When did Bill Shankly's resignation from Carlisle United happen?