The "hour of mercy" is reckoned by Tradition at 3:00 in the afternoon, the moment when Jesus Christ died and gave His life as ransom for many. It is the time when parishes begin the Good Friday Liturgy. I have come across a rare case or two when the announcement of His death as told in the Passion Narrative of St. John was timed so as to be at that exact moment.
It is also the time when the Chaplet of Divine Mercy is recited. I refer you to my first post on this subject (a week short of six years ago) for details about this devotion.
For those who practice this, no reminder is needed. For those who wish to practice it, this reminder is for you.