All Saints Day is one of the biggest feasts of the year.
It's the day when we celebrate all of God's saints, known and unknown and we seek their intercession. In the Catholic Church, it's a holy day of obligation (Select days throughout the year that are not on Sunday where attendance at Mass is necessary). Historically, All Saints Day involved at least one Mass (probably more) and a great deal of feasting. In recent years, it has been overshadowed in common culture by the celebration of Halloween, which was originally also a part of All Saints Day, though then it was known as All Hallows Eve.
All Saints Day is a feast in which we celebrate the Church Triumphant, asking their intercession for us, the Church Militant. The saints know the struggles of living a holy life. When we ask them to intercede for us, they do their best to help us. This is one of the greatest aspects of the Catholic Church in my opinion. Those in Heaven are not cut off from us; they still offer their aid and do so happily.