Sorry, I used one of many links that endorsed Ash Wednesday to Holy Thursday as being the period for Lent.
I could very well be reading the documents incorrectly. However, if I am in fact doing so, in "General Norms for the Liturgical Year and the Calendar" (one of many sources found here:
http://www.catholicliturgy.com/index.cf ... 1/Start/97), which states:
28. Lent runs from Ash Wednesday until the Mass of the Lord's Supper exclusive.
This document is addressed by Pope Paul VI through "Mysterii paschalis", here:
http://www.vatican.va/holy_father/paul_ ... is_en.html, and at least to me reads as an endorsement, and in fact in some sources the two documents are presented together:
http://www.liturgyoffice.org.uk/Calendar/Info/GNLY.pdf
The United States Conference of Catholic Bishops also addresses this here:
http://www.usccb.org/prayer-and-worship ... t-lent.cfm; they extend the Lenten designation to Holy Saturday, but they too acknowledge that Lent extends past Palm Sunday (stating that the 40 days of prayer and fasting exclude Sundays, however the Sundays are not fully "exempt" per se).
Other support for Holy Thursday as the end of Lent:
Archdiocese of Toronto:
http://www.archtoronto.org/events_news/lent.html
Archdiocese of Boston:
http://www.bostoncatholic.org/uploadedF ... elines.pdf (on page 7)
Also supported on the official websites of the Dioceses of Grand Rapids, Wichita, Providence, and Orlando on the first page of a Google search.
I very well could be reading everything incorrectly; I'm not a Church scholar and (obviously from my first attempt) I am not the best at compiling and utilizing sources. But to me, the literature I have read endorses Lent at the very least extending into Holy Week. I found very few resources that support Palm Sunday as the end of Lent.