All Saints' Day - A Holy Day of Obligation

The communion of saints is the spiritual solidarity which binds together the  faithful on earth, the  souls in  purgatory, and the saints in  heaven in the organic unity of the same  mystical body under Christ its head, and in a constant interchange of  supernatural offices. The participants in that solidarity are called saints by reason of their destination and of their partaking of the fruits of the  Redemption ( 1 Corinthians 1:2 — Greek Text). The damned are thus excluded from the communion of saints. The living, even if they do not belong to the body of the  true  Church, share in it according to the measure of their union with Christ and with the  soul of the  Church. (http://www.newadvent.org/cathen/04171a.htm)