Since the 8th century, the feast of All Saints has been set in the West on Nov. 1. All Souls is Nov. 2--the day to pray for all the dead, like the Eastern Meatfare Saturday.
Cool. How would the vespers service on October 31 go?
It would depend on which rite it is (Gregorian, Tikhonian, Sarum, Benedictine, Mozarabic, Ambrosian, Celtic...)
Benedictine vespers starts with Psalms, which are heralded by special festal antiphons like:
Antiphon 1: I beheld a great multitude which no man could number, of all nations, standing before the throne. (The Western rites use Revelation liturically for its beautiful imagery.)
Then there is the hymn "Christe Redeptor omnium," which asks for the intercessions of all the saints.
And then the collect: "Almighty everlasting God, by whose favor we venerate the merits of all thy Saints in one solemnity: we beseech thee to bestow upon us, through so many intercessors, the abundance of thy mercy which we desire."
Meritas in Latin needs to be understood separately from the Reformation era argument, as it predates it by many centuries.