"All Saints Day", also knows as "All Hallows", is celebrated ever year on November 1st to celebrate all of the souls in heaven (Saints), both known and unknown. It is followed by "All Souls Day" on November 2nd which is celebrated for all those who are awaiting Heaven in purgatory.
In the Roman Catholic Church "All Saints Day" is a "Holy Day of Obligation". With Sundays being the clearest example, there are specific days during the year in which the Church asks us to attend Mass called "Holy Days of Obligation".
For the history of this Holy Day, Wikipedia provides an excellent step-by-step account.
A great prayer from the "Liturgy of the Hours", a Catholic form of daily prayer, from "All Saints Day" is:
Father, All-Powerful and ever-living God,
today we rejoice in the holy men and women
of every time and place.
May their prayers bring us your forgiveness and love
We ask this in the name of Jesus Christ our Lord. Amen.