The Feast of the Immaculate Conception always is on December 8th. It celebrates the belief that Mary, mother of Jesus, never sinned in all her life.
In Colombia it is traditionally celebrated with a feast for the Virgin Mary and a mandatory church service. This day can be taken off, but isn't a mandatory date to celebrate.
This holiday is based off the bible who says that Mary's mother was St. Anne to which and angel had spoken to her to tell her that her child would be honored by the world. It celebrates the birth of Mary, but many scholars argued if they should make it a holiday and it wasn't until Pope Pious IX proclaimed this belief to be an essential dogma of the Roman Catholic Church that it became a national holiday in 1854.