Los Dias De Los Muertos is plural, and refers to the dates Oct. 31st through November 1st. In English, those three days are referred to as All Saints (or All Hallows) Eve, All Saints Day, and All Souls Day.
El Dia De Los Muertos is the singular form, and refers specifically to November 2nd, the day to celebrate all the departed souls, not just those who the church has stuck a fancy halo on.
Eternal Rest grant unto them, O Lord,
and let perpetual light shine upon them.
May they rest in peace and rise in glory.
May those for whom grief is still an open wound