E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To appease his Mnes. To do when a person is dead what would have pleased him or was due to him when alive. The spirit or ghost of the dead was by the Romans called his Manes, which never slept quietly in the grave so long as survivors left its wishes unfulfilled. The 19th February was the day when all the living sacrificed to the shades of dead relations and friends.
Manes (2 syl.) from the old word manis, i.e. bonus quod eos venerantes manes vocarent, ut Græci chrstous. (See Lucretius, iii. 52.) It cannot come from mneo, to remain (because this part of man remains after the body is dead), because the a is long.
In the Christian Church there is an All Souls Day.