The novena began on Good Friday, but it's not too late to start.
The Divine Mercy novena , revealed by our Lord to St. Faustina Kowalska , was one of the favorite devotions of the Servant of God Pope John Paul II. He promulgated Divine Mercy Sunday as a feast of the Universal Church with this homily in 2001. He said that was "the happiest day of my life".
Four years later, our Lord called his servant home on the eve of Divine Mercy Sunday, April 3, 2005.
For those who reject Divine Mercy Sunday as a novelty, this essay explains how celebrating a feast on the Octave of Easter goes back to the earliest liturgical writings of the Apostolic Fathers.