Fasting literally means to "not eat".

Why, you ask, would someone intentionally "not eat"?  What purpose could it possibly serve?  Leviticus 16:29 tells us that fasting is tied to spirituality. 

Leviticus 16:29:  "And this shall be a statute for ever unto you: that in the seventh month, on the tenth day of the month, ye shall afflict your souls, and do no work at all, whether it be one of your own country, or a stranger that sojourneth among you:"   

We further understand what it means to "afflict your soul" when we couple this vers with Psalm 35:13:  "But as for me, when they were sick, my clothing was sackcloth: I humbled my soul with fasting; and my prayer returned into mine own bosom.", and with Psalm 69:10:  "When I wept, and chastened my soul with fasting, that was to my reproach." 

Fasting is more than just "afflicting one's body". It is "afflicting one's soul."   Fasting is a way of expressing that food and self desires are secondary to something else! Fasting is "afflicting one's soul" -- an act of self-denial.

Biblical fasting is the act of "not eating" with spiritual communication in mind. How do we know this? Because Biblical fasting always occurs together with prayer.  You can pray without fasting, but you cannot fast (not according to the bible) without praying. Biblical fasting is deliberately abstaining from food for a spiritual reason: communication and relationship with God! 

The Bible gives examples of many different kinds of fasting.

There are actually very few instructions when it comes to fasting.  What you do for a fast is  between you and the Lord. There is only one full and complete fast commanded in the Bible and that was the fast on the Day of Atonement. This fast was from sunset of one day to sunset of the next, for 24 full hours. (Leviticus 16:29;23:32). Since in those days people usually don't eat during the night that makes the fast fairly easy, since you can eat again just before going to bed the next evening (if that's what they wished to do).  According to Jewish rabbis:  the Mishna, Yoma 8:1 states:  "on the Day of Atonement it is forbidden to eat, or drink, or bathe, or anoint oneself, or wear sandals, or to indulge in conjugal intercourse".  This direction is not from the Bible, but perhaps we can look at that as our instruction for a typical fast since fasting was given by God to the Jewish people.  So in this type of fast the person abstained from food and liquid for a 24 hour period of one day (from sunset to sunset). This is a normal fast. 

There was also a partial fast. In this type of fast, the emphasis is placed on refraining from certain foods, rather than abstaining completely from eating. Daniel, Shadrach, Meshach and Abednego did this by eating only vegetables and drinking only water (Daniel 1:15).  We read that Daniel did a similar thing later in the book:  Daniel 10:3 "I ate no pleasant bread, neither came flesh nor wine in my mouth, neither did I anoint myself at all, till three whole weeks were fulfilled."   

Many argue that this isn't really a fast at all, because Daniel didn't refrain from eating completely, but when you consider Daniel 10:2:   "In those days I Daniel was mourning three full weeks." it is easier to see that this was indeed a fast.  Mourning in the bible is associated with fasting. 

The last type of fast is one that we don't often see today.  The extended fast is one in which a person refrains from both food and water OR from simply food (but not water) for an extended period of time.  

Warning:  This type of fast can be harmful to your health and in most cases should not exceed three days.  

Esther decided to fast for three days abstaining from both "food and water" both "day and night" (Esther 4:15-16) in order to seek God's will for an entire nation. The rApostle Paul also went without food and water for three days after encountering Jesus on the road to Damascus. (Acts 9:9).  

King David fasted seven days as a plea to God to save the life of his child (2 Samuel 12:15-20).  

Fasts that extend beyond three days (sometimes seven days) can be found in the Bible, but these are the exceptions, not the rule.   

Moses, Elijah, and Jesus all fasted for 40 days. 

Why should we fast today?

We fast as a way to seek God will all our heart:  Jeremiah 29:13-14   "And ye shall seek me, and find me, when ye shall search for me with all your heart.  And I will be found of you, saith the LORD: and I will turn away your captivity, and I will gather you from all the nations, and from all the places whither I have driven you, saith the LORD; and I will bring you again into the place whence I caused you to be carried away captive."  

I believe whey you are willing to afflict your body and to restrain yourself from the body's demand for food, it is a sign that you are truly ready to get to seek God with all your heart.   

Fasting is a a way to show God that you are seeking him with all your heart.  It is a direct response to God's call to return to him:  Joel 2:12-13 "Therefore also now, saith the LORD, turn ye even to me with all your heart, and with fasting, and with weeping, and with mourning: And rend your heart, and not your garments, and turn unto the LORD your God: for he is gracious and merciful, slow to anger, and of great kindness, and repenteth him of the evil."   

The question is often asked:   "How do I know when to pray and fast and when to just pray?" 

That's not a question that is easily answered and it is not a question that someone else can answer for you!  The only thing I can do is give you a very basic guidline from what we have seen in the Word of God.  

In every case of Biblical fasting we see a couple of things:   

(1)  Fasting is connected with a deeply trouble spirit.

(2)  Fasting is connected with an anxious heart before God. 

Why would we fast? To demonstrate that we are seeking God "with all our heart." Fasting puts things in proper focus for us (not to mention that there are medical benefits). It is a physical way of saying, "Food and the things of this life are not as important to me now as getting close to the Lord Jesus ."

Fasting also shows humility (although this is negated if you do it for an open show.)We have already cover the passages that deal with this fact: (Psalm 35:13; 1 Kings 21:29; Ezra 8:21). When a person is really concerned about the things of God, he will humble himself. There will be times though, when he will abstain from the enjoyment of food to continue concentration and focus on the things that are important to God