A Tuesday
Uther has been assassinated before s3e12?. Morgana has been enchanted to follow Morgause. Merlin manages to break the spell right before Morgause attacks Camelot with an army of sorcerers and essetirians, but some guards and knights are killed and Arthur is mortally wounded.
Merlin uses the magical matrix of the Infinity stones as the base of a spell that translates roughly to
"Here (Space)
Now (Time)
Those that were taken (Soul)
As they stand eternal (Mind)
Breaking the laws of space, time and magic (Power)
Balance of fates, let it be my command (Reality)",
which kills Morgause and the essetirians and frees the sorcerers from an obedience spell, similar to one placed on Morgana, and also revives some dead people and heals Arthur.
Arthur is shocked and says "you saved my life... With magic".
Merlin snorts and replies "must be a Tuesday".