Candlemas, aka The Purification of Our Lady, aka the Presentation of Jesus at the Temple, is celebrated 40 days after Christmas, the traditional date for the ceremonies of purification after childbirth in Jewish law that were translated over into Catholic tradition as the rites for the churching of women.
In Jewish tradition women were ritually impure after childbirth; in Catholic tradition the ceremony is a thanksgiving for survival of childbirth, and a blessing for the future. In both cases it was no doubt a practical measure in part, to allow the mother to recover before having to resume her normal duties including attending worship! These days of course the idea is considered ideologically unsound, so the feast has been renamed the Presentation of Our Lord in the Temple (which was certainly part of the traditional ceremonies, albeit the part supplanted by infant baptism).
The popular name for the feast comes from the ceremony held on the day whereby the candles to be used for the year ahead are blessed.
The Gospel for the day, Luke 2, describes the events, and gives us the Nunc Dimittis, Simeon's canticle used at Compline in the Roman Rite.