The fast, of course, begins today. For a recent thread on its history and purpose, check this out: http://www.orthodoxchristianity.net/forum/index.php?topic=8905.0
Other than that, I must say that I was struck last night and this morning by the obvious and pervasive liturgical shift that comes with the end of the Pentecostarion. For months now, we have been chanting special hymns on weekdays, special hymns on the weekend, special hymns during the Liturgy. But, as soon as the last set of special hymns, the Pentecostarion (which is filled with talk of resurrection and joy), as soon as this period ends, we enter a fast.
We don't waste a single day. No longer do we chant special verses, no longer do we have different katavasies or apolytikia/kontakia. Our many-weeks-long preparation for Pascha is over, and our many-weeks-long celebration of Pascha has come to an end.
And now, immediately, to the fast!