The second Sunday of Easter, while also designated Divine Mercy Sunday, remains the conclusion of the Octave of Easter. In other words, Divine Mercy is not something that happens subsequent to Easter; rather, Divine Mercy happens within Easter.
A translation of today’s Collect (by Fr Zuhlsdorf):
O God of eternal mercy,
who on this recurrence of the paschal feast
do kindle the faith of a people sanctified for Yourself,
increase the grace which You have given,
so that all may comprehend with worthy understanding
by what laver they were washed,
by what Spirit they were regenerated,
by what Blood they were redeemed.