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

O God, when you went out before your | people,*
when you marched through the wilderness, Alle- | luia
the earth quaked, the heavens poured down rain, Alle- | luia*
Alle- | luia

God shall arise, his enemies shall be | scattered;*
and those who hate him shall flee be- | fore him!
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.

O God, when you went out before your | people,*
when you marched through the wilderness, Alle- | luia
the earth quaked, the heavens poured down rain, Alle- | luia*
Alle- | luia


 

 

 

The Collect of the Day

May the Comforter who proceeds from Thee, enlighten our minds, we beseech Thee, Ò Lord: and guide us, as Thy Son has promised, into all trùth; through the same Jesus Christ, Thy Son, oùr Lord, who liveth and reigneth with Thee in the unity of the same Hòly Ghost: ever one God, world without end. Amen.

 
St. Peter Repentant,   Mid- 17th century, by Ludolph Büsinck, German from the  Philadelphia Museum of Art  

St. Peter Repentant, Mid- 17th century, by Ludolph Büsinck, German from the Philadelphia Museum of Art 

The First Lesson 

Acts 2:14-21

But Peter, standing up with the eleven, raised his voice and said to them, “Men of Judea and all who dwell in Jerusalem, let this be known to you, and heed my words. For these are not drunk, as you suppose, since it is only the third hour of the day. But this is what was spoken by the prophet Joel:

‘And it shall come to pass in the last days, says God,
That I will pour out of My Spirit on all flesh;
Your sons and your daughters shall prophesy,
Your young men shall see visions,
Your old men shall dream dreams.
And on My menservants and on My maidservants
I will pour out My Spirit in those days;
And they shall prophesy.
I will show wonders in heaven above
And signs in the earth beneath:
Blood and fire and vapor of smoke.
The sun shall be turned into darkness,
And the moon into blood,
Before the coming of the great and awesome day of the Lord.
And it shall come to pass
That whoever calls on the name of the Lord
Shall be saved.’

 

The Verse: By the word of the LORD the heavens were made,
and by the breath of his mouth all their host.

 

The Epistle

Acts 5:12-16

And through the hands of the apostles many signs and wonders were done among the people. And they were all with one accord in Solomon’s Porch. Yet none of the rest dared join them, but the people esteemed them highly. And believers were increasingly added to the Lord, multitudes of both men and women, so that they brought the sick out into the streets and laid them on beds and couches, that at least the shadow of Peter passing by might fall on some of them. Also a multitude gathered from the surrounding cities to Jerusalem, bringing sick people and those who were tormented by unclean spirits, and they were all healed.

 
Melchisidech Receiving Bread and Wine  , 18th century, by Giovanni Antonio Faldoni, Italian, from the  Philadelphia Museum of Art  

Melchisidech Receiving Bread and Wine, 18th century, by Giovanni Antonio Faldoni, Italian, from the Philadelphia Museum of Art 

The Holy Gospel

 John 6:44-52

Alleluia. Alleluia. Come, Holy Spirit, fill the hearts of the faithful and kindle in them the fire of Thy love. Alleluia. Alleluia.

No one can come to Me unless the Father who sent Me draws him; and I will raise him up at the last day. It is written in the prophets, ‘And they shall all be taught by God.’ Therefore everyone who has heard and learned from the Father comes to Me. Not that anyone has seen the Father, except He who is from God; He has seen the Father. Most assuredly, I say to you, he who believes in Me has everlasting life. I am the bread of life. Your fathers ate the manna in the wilderness, and are dead. This is the bread which comes down from heaven, that one may eat of it and not die. I am the living bread which came down from heaven. If anyone eats of this bread, he will live forever; and the bread that I shall give is My flesh, which I shall give for the life of the world.”

The Jews therefore quarreled among themselves, saying, “How can this Man give us His flesh to eat?”