In the Roman Rite, Lent officially begins on Ash Wednesday. This day marks the start of a 40-day period of fasting, prayer, and penance leading up to Easter. The ashes used on Ash Wednesday are a symbol of repentance and mortality, and they are typically placed on the foreheads of the faithful in the shape of a cross.
Lent ends at the beginning of the Mass of the Lord's Supper on Holy Thursday evening. This Mass marks the start of the Easter Triduum, which is a separate liturgical season that includes Good Friday, Holy Saturday, and Easter Sunday. The Triduum is considered the most solemn part of the Christian liturgical year, culminating in the celebration of the Resurrection of Jesus on Easter Sunday.
For more detailed information, you can refer to the Catholic Answers article on Lent and the USCCB's explanation of Lent.