The mass for Divine Mercy is the Sunday after Easter Sunday. I’ve recently been told that if you go to confession during this week after Easter, your past punishments (referring to purgatory) will be forgiven. Is this true?
This is called an “indulgence”. You do not receive the indulgence for going to Mass on this Sunday. Going to Mass on a Sunday is a requirement for Catholics. From the 1968 document Enchiridion Indulgentiarum: “31. An indulgence cannot be attached to a work to which a person is obliged by law or precept.”
Instead particular prayers need to be said on this day:
“… on the Second Sunday of Easter or Divine Mercy Sunday, in any church or chapel, in a spirit that is completely detached from the affection for a sin, even a venial sin, take part in the prayers and devotions held in honour of Divine Mercy, or who, in the presence of the Blessed Sacrament exposed or reserved in the tabernacle, recite the Our Father and the Creed, adding a devout prayer to the merciful Lord Jesus (e.g. Merciful Jesus, I trust in you!”);"
Going to confession is not required during this week after Easter. It can be later. From Enchiridion Indulgentiarum:
“26. The requirements for gaining a plenary indulgence are the performance of the indulgenced work and fulfillment of three conditions: sacramental confession, eucharistic communion, and prayer for the pope’s intentions. A further requirement is the exclusion of all attachment to sin, even venial sin. …
Unless this unqualified disposition and the three conditions are present, the indulgence will only be partial; …
27. The three conditions may be carried out several days preceding
or following performance of the prescribed work.”
Praying for the pope’s intentions is described:
“29. The condition requiring prayer for the pope’s intentions is completely satisfied by reciting once the Our Father and Hail Mary for his intentions; nevertheless all the faithful have the option of reciting any other prayer suited to their own devotion and their reverence for the pope.”
The decree for the Divine Mercy indulgence (I quoted part of above) is at
vatican.va/roman_curia/tribunals/apost_penit/documents/rc_trib_appen_doc_20020629_decree-ii_en.html .
More about indulgences can be learnt from this 1967 document by Pope Paul VI:
vatican.va/holy_father/paul_vi/apost_constitutions/documents/hf_p-vi_apc_19670101_indulgentiarum-doctrina_en.html
The parts of Enchiridion Indulgentiarum I quoted above are from this book: Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4, pages 1014-1015.