John 9

I remember very clearly a comment made by a member of the stake presidency as I served on the stake high council. He suggested that a window of opportunity had now opened and the day would come when that window would close. When the window closed, our opportunity to do any more would also close. Today was the day that we must do all we can before the window of opportunity would not available anymore.

The Savior seemed to be more than aware of this doctrine, teaching -

"I must work the works of him that sent me, while it is day: the night cometh, when no man can work." (New Testament | John 9:4)

Alma taught much the same concept -

"For behold, this life is the time for men to prepare to meet God; yea, behold the day of this life is the day for men to perform their labors." (Book of Mormon | Alma 34:32)

"...I beseech of you that ye do not procrastinate the day of your repentance until the end; for after this day of life, which is given us to prepare for eternity, behold, if we do not improve our time while in this life, then cometh the night of darkness wherein there can be no labor performed." (Book of Mormon | Alma 34:33)

In both these sayings, it is referring to life itself. But I believe it also applies to every window of opportunity or season of service fro the day will come when "can be no labor performed".