The Solemnity of the Assumption of the Blessed Virgin Mary is celebrated on August 15th.
What is the Assumption of the Blessed Virgin Mary?
The doctrine of the Assumption says that at the end of her life on earth Mary was assumed, body and soul, into heaven, just as Enoch, Elijah, and perhaps others had been before her. It’s also necessary to keep in mind what the Assumption is not. Some people think Catholics believe Mary "ascended" into heaven. That’s not correct. Christ, by his own power, ascended into heaven. Mary was assumed or taken up into heaven by God. She didn’t do it under her own power.
The possibility of a bodily assumption before the Second Coming is suggested by Matthew 27:52–53: "[T]he tombs also were opened, and many bodies of the saints who had fallen asleep were raised, and coming out of the tombs after his resurrection they went into the holy city and appeared to many." Did all these Old Testament saints die and have to be buried all over again? There is no record of that, but it is recorded by early Church writers that they were assumed into heaven, or at least into that temporary state of rest and happiness often called "paradise," where the righteous people from the Old Testament era waited until Christ’s resurrection (cf. Luke 16:22, 23:43; Heb. 11:1–40; 1 Pet. 4:6), after which they were brought into the eternal bliss of heaven.
Source: catholic.com