[Mike Fieschko]

Is it supposed to somehow absolve them of all the legal and moral wrongdoing they have done all year?

No.

A Mass never does that, for anyone or any mortal/grave sin.

Only individual confession for each person would do that, provided all the requirements for a confession are present (genuine contrition, firm purpose of amendment, etc.).

[sneakypete]

No.

A Mass never does that, for anyone or any mortal/grave sin.

Thanks for the explanation.