Michammi dukadhamis an ancient Prakrit phrase literally meaning – may all the evil that has been done be fruitless. It is especially used on the Kshamavani Diwas or Forgiveness Day, celebrated on Samvatsari, the concluding day of the eight or ten day Paryushana festival, one of the main festivals of the Jain community. On this day, Jains request forgiveness from each other for all offences committed. The phrase is also used when a person makes a mistake, or recollects making one in everyday life, or when asking for forgiveness in advance for inadvertent ones.

As a matter of ritual, Jains greet their friends and relatives with Micchami Dukkadam, seeking their forgiveness. Forgiveness is requested by saying “Micchami Dukkadam” to each other. It means “If I have caused you offence in any way, knowingly or unknowingly, in thought, word or deed, then I seek your forgiveness”. No private quarrel or dispute may be carried beyond Samvatsari, and traditionally, letters have been sent and telephone calls made to friends and relatives asking their forgiveness.

Source: https://en.wikipedia.org/wiki/Micchami_Dukkadama