Ash Wednesday and Good Friday are days of fast and abstinence. All the Fridays in between are days of abstinence. For guidelines see here.
READINGS AND OFFICE:
Today’s Mass Readings (NABRE). Used in the U.S.A.
Today’s Mass Readings (NJB). Used in most English speaking countries
COMMENTARIES ON THE FIRST READING: Joel 2:12-18.
COMMENTARIES ON THE RESPONSORIAL: Psalm 51:3-4, 5-6ab, 12-13, 14 and 17.
St John Fisher’s Sermons on Psalm 51. Psalm 50 in Fisher’s translation. The Fourth Penitential Psalm. He treated of the Psalm in two parts, and at some length.
COMMENTARIES ON THE SECOND READING: 2 Corinthians 5:20-6:2.
COMMENTARIES ON THE GOSPEL: Matthew 6:1-6, 16-18.
GENERAL RESOURCES: sites that usually deal with the readings as a whole (with some occasional specialty studies). Commentaries on individual readings are listed further below.
Unofficial Lectionary. Readings from the Douay-Rheims Challoner version followed by notes from the old Haydock Commentary.
Sacerdos. Gives the theme of the readings, the doctrinal message, and pastoral application.
Lector Notes. Brief historical and theological background on the readings. Can be printed out, copied, and used as bulletin insert.
The Bible Workshop. Links to several relevant articles, contains a reading guide to the gospel text, a comparison of the readings, suggestions for a lesson (i.e., homily).
St Charles Borromeo Parish Study Notes. Notes on all the readings, usually with some background info as well.