Lent occurs between the periods of Ash Wednesday and Easter.  Technically, Lent is 40 days long, though it occurs over 46 calendar days.  This is because there are 40 days of fasting along with 6 Sundays. 

The 40 days of Lent are symbolic of the 40 day period in which Jesus spent in the desert being tempted by Satan.  
As the Gospel of Matthew (Chapter 4) says, 

"4 Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil. ² He fasted forty days and forty nights, and afterwards he was famished. ³ The tempter came and said to him, “If you are the Son of God, command these stones to become loaves of bread.” ⁴ But he answered, “It is written,

‘One does not live by bread alone,
    but by every word that comes from the mouth of God.’”

⁵ Then the devil took him to the holy city and placed him on the pinnacle of the temple, ⁶ saying to him, “If you are the Son of God, throw yourself down; for it is written,

‘He will command his angels concerning you,’
    and ‘On their hands they will bear you up,
so that you will not dash your foot against a stone.’”

⁷ Jesus said to him, “Again it is written, ‘Do not put the Lord your God to the test.’”

⁸ Again, the devil took him to a very high mountain and showed him all the kingdoms of the world and their splendor; ⁹ and he said to him, “All these I will give you, if you will fall down and worship me.” ¹⁰ Jesus said to him, “Away with you, Satan! for it is written,

‘Worship the Lord your God,
    and serve only him.’”

¹¹ Then the devil left him, and suddenly angels came and waited on him."

NRSVCE