Our Lady in Medjugorje speaks to you about fasting and how to have thanks

Message dated August 31, 1981
In order for that sick child to heal, his parents must firmly believe, pray earnestly, fast and do penance.
Some passages from the Bible that can help us understand this message.
Isaiah 58,1-14
She screams at the top of her mind, have no regard; like a trumpet, raise your voice; he declares his crimes to my people, his sins to the house of Jacob. They seek me out every day, yearning to know my ways, like a people who practice justice and have not abandoned the right of their God; they ask me for just judgments, they crave the closeness of God: "Why fast, if you don't see it, mortify us, if you don't know it?". Behold, on the day of your fast you take care of your affairs, torment all your workers. Here, you fast between quarrels and altercations and hitting with unfair punches. Do not fast any more as you do today, so that your noise can be heard high up. Is the fasting that I crave like this the day on which man mortifies himself? To bend one's head like a rush, to use sackcloth and ashes for the bed, perhaps you would like to call fasting and a day pleasing to the Lord?

Isn't this the fast I want: to untie the unfair chains, remove the bonds of the yoke, put the oppressed free and break every yoke? Does it not consist in sharing bread with the hungry, in introducing the poor, homeless into the house, in dressing someone you see naked, without taking your eyes off those of your flesh? Then your light will rise like the dawn, your wound will heal soon. Your righteousness will walk before you, the glory of the Lord will follow you. Then you will call on him and the Lord will answer you; you will beg for help and he will say, "Here I am!" If you take away the oppression, the pointing of the finger and the ungodly speaking from among you, if you offer the bread to the hungry, if you satisfy the fast, then your light will shine in the darkness, your darkness will be like the noon. The Lord will always guide you, he will satisfy you in arid soils, he will reinvigorate your bones; you will be like an irrigated garden and a spring whose waters do not dry up. Your people will rebuild the ancient ruins, you will rebuild the foundations of distant times. They will call you breccia repairman, restorer of ruined houses to live in. If you refrain from violating the Sabbath, from carrying out business on the day sacred to me, if you will call the Sabbath delight and venerating the holy day to the Lord, if you will honor it by avoiding to set off, to do business and to bargain, then you will find the delight in the Lord. I will make you tread the heights of the earth, I will make you taste the legacy of Jacob your father, since the mouth of the Lord has spoken.
Sirach 10,6-17
Do not worry about your neighbor for any wrong; do nothing in anger. Pride is hateful to the Lord and to men, and injustice is abominable to both. The empire passes from one people to another because of injustice, violence and wealth. Why on earth is it proud who is earth and ash? Even when alive his bowels are repugnant. The illness is long, the doctor laughs at it; whoever is king today will die tomorrow. When man dies he inherits insects, beasts and worms. The principle of human pride is to get away from the Lord, to keep one's heart away from those who created it. In fact, the principle of pride is sin; whoever abandons himself spreads the abomination around him. This is why the Lord makes his punishments incredible and scourges him to the end. The Lord has brought down the throne of the powerful, in their place has made the humble sit. The Lord has uprooted the roots of the nations, in their place has planted the humble. The Lord has upset the regions of the nations, and has destroyed them from the foundations of the earth. He uprooted and annihilated them, he made their memory disappear from the earth.