Why Doesn't Lent Begin on the same date each year?

Lent always immediately precedes Easter, and the date of Easter is not a fixed date, but is celebrated on the first Sunday after the first full moon after the first day of spring (the vernal equinox).

Why Does Lent Last 40 days?

Jesus spent 40 days in the wilderness preparing for His formal ministry, so Lent uses the model of 40 days (Sundays don't count) to signify tht this is a period of preparation, a time when we ready our hearts to remember Jesus' suffering and death for our salvation.