Because tomorrow is Holy Cross Day, this Wednesday, Friday and Saturday are the fall Ember days. You can read more about Ember days on page 21 in the Treasury. Note that in Lutheran use these fasting days were devoted also to the Catechism, so a great way to observe the Ember days is to review the entirely of the six chief parts from the Small Catechism as you fast*:
Wednesday: Ten Commandments and Creed
Friday: Lord's Prayer and Holy Baptism
Saturday: Confession and Sacrament of the Altar
There are also traditional readings associated with these days, provided by the Brotherhood Prayer Book:
Wednesday: Mark 9:16-28; Ezra 8:1-10
Friday: Luke 7:36-50; Hosea 14:2-10
Saturday: Luke 13:6-17; Hebrews 9:2-12
Also the traditional collects:
Wednesday: Vouchsafe, O Lord, we pray thee, to aid our infirmities with the healing of Thy loving-kindness: that we, which are made weak by the frailty of our nature, may be strengthened by the power of Thy mercy; through Jesus Christ...
Friday: Grant, we beseech Thee, almighty God: that we, who put our trust in Thee, may serve Thee acceptably both in body and soul; through Jesus Christ...
Saturday: Almighty and everlasting God, who dost command bodily discipline and hast attached promises thereto: grant that we may constantly serve Thee in soul and body, that no temptation may cause us to stumble; through Jesus Christ...
[See Brotherhood Prayer Book, pp. 525, 526]
* For a fine article on fasting, see The Lutheran Study Bible on p. 189 "When You Fast." Note especially: "Yet our Lord's words clearly reveal that fasting should be part of a Christian's life. He said, 'When you fast,' not 'if you fast.' The early Christians fasted (Acts 13:2-3; 14:23). Why shouldn't a twenty-first century Christian do likewise? As you fast, let the feelings of hunger you experience remind you to pray. Spend the time you would normally spend eating by reading God's Word and meditating on the Gospel of Jesus Christ. Through His Word the Lord will bless and nourish you."