This suggests that the heavenly Bridegroom will not return as soon as the church expects him. We have a hint of the same kind in Matt. They all slumbered and slept. The thought is that the Bridegroom came at an unexpected moment. And at midnight there was a cry made.
Midnight is the hour of slumber and silence when an outcry is always startling and unexpected.
The implication is that the cry of the coming of the Bridegroom, the trump of the archangel, will be sudden and unexpected. All . . . arose, and trimmed their lamps. The object of trimming would be to secure a brighter light. It would involve the trimming of the wick and replenishing the oil if needed. When the cry does arise there will be a wonderful lamp-trimming.