Ember days - from the Latin "QuattuorTempora" meaning "Four Seasons" - are traditional days of fasting and almsgiving for Western Christians.
The Four Embertides coincide with changes in the seasons and have been used by farmers to determine planting and harvesting times. Ember days fall on the Wednesday, Friday, and Saturday of the prescribed week:

Winter: St. Lucy Day (December 13) 
Spring: Invocabit, The first Sunday in Lent
Summer:  Pentecost
Fall: Holy Cross Day (September 14)

A Lutheran suggestion to the practice of fasting can be found here 
These readings are meant to be part of the Divine Service, but of themselves serve as a good addition to fasting and almsgiving on Ember Days.

 

The Introit

My mouth is filled with your praise, Alle- | luia*
My lips will shout for joy, when I sing praises to you; Alle- | luia* 
Alle- | luia

In you, O LORD, do I take | refuge;*
let me never be | put to shame!
In your righteousness deliver me and | rescue me;*
incline your ear to me, and | save me!
Glory be to the Father and to the Son
and to the Holy Spirit;
as it was in the beginning,
is now, and will be forever. Amen.

My mouth is filled with your praise, Alle- | luia*
My lips will shout for joy, when I sing praises to you; Alle- | luia* 
Alle- | luia

The Collect of the Day

Grant to Thy Church, we beseech Thee, O mercìful God: that being gathered together in the Holy Spirit, she may in no way be troubled by any hostile attàcks; through Jesus Christ, Thy Son, oùr Lord, who liveth and reigneth with Thee in the unity of the same Holy Spìrit: ever one God, world without end. Amen.

 
The Prophet Joel  , c. 1572, by Giorgio Ghisi, Italian from the  Philadelphia Museum of Art  

The Prophet Joel, c. 1572, by Giorgio Ghisi, Italian from the Philadelphia Museum of Art 

The First Lesson 

Joel 2:23-24, 26-27

Be glad then, you children of Zion,
And rejoice in the Lord your God;
For He has given you the former rain faithfully,
And He will cause the rain to come down for you—
The former rain,
And the latter rain in the first month.
 The threshing floors shall be full of wheat,
And the vats shall overflow with new wine and oil.
You shall eat in plenty and be satisfied,
And praise the name of the Lord your God,
Who has dealt wondrously with you;
And My people shall never be put to shame.
Then you shall know that I am in the midst of Israel:
I am the Lord your God
And there is no other.
My people shall never be put to shame.

 
Christ Among the Doctors  , 16th century, Copy after Hieronymus Bosch, Netherlandish from the  Philadelphia Museum of Art  

Christ Among the Doctors, 16th century, Copy after Hieronymus Bosch, Netherlandish from the Philadelphia Museum of Art 

The Holy Gospel

Luke 5:17-26

Alleluia. Alleluia. O how good and sweet is thy spirit, O Lord, in all things! Alleluia. Come, Holy Spirit, fill the hearts of the faithful and kindle in them the fire of Thy love.  Alleluia.

Now it happened on a certain day, as He was teaching, that there were Pharisees and teachers of the law sitting by, who had come out of every town of Galilee, Judea, and Jerusalem. And the power of the Lord was present to heal them. Then behold, men brought on a bed a man who was paralyzed, whom they sought to bring in and lay before Him. And when they could not find how they might bring him in, because of the crowd, they went up on the housetop and let him down with his bed through the tiling into the midst before Jesus.

When He saw their faith, He said to him, “Man, your sins are forgiven you.”

And the scribes and the Pharisees began to reason, saying, “Who is this who speaks blasphemies? Who can forgive sins but God alone?”

But when Jesus perceived their thoughts, He answered and said to them, “Why are you reasoning in your hearts? Which is easier, to say, ‘Your sins are forgiven you,’ or to say, ‘Rise up and walk’? But that you may know that the Son of Man has power on earth to forgive sins”—He said to the man who was paralyzed, “I say to you, arise, take up your bed, and go to your house.”

Immediately he rose up before them, took up what he had been lying on, and departed to his own house, glorifying God. And they were all amazed, and they glorified God and were filled with fear, saying, “We have seen strange things today!”