  We report a 5.4\sigma\ detection of pulsed gamma rays from PSR B1821-24 in
the globular cluster M28 using ~44 months of Fermi Large Area Telescope (LAT)
data that have been reprocessed with improved instrument calibration constants.
We constructed a phase-coherent ephemeris, with post-fit residual RMS of 3 \mu
s, using radio data spanning ~23.2 years, enabling measurements of the
multi-wavelength light curve properties of PSR B1821-24 at the milliperiod
level. We fold RXTE observations of PSR B1821-24 from 1996 to 2007 and discuss
implications on the emission zones. The gamma-ray light curve consists of two
peaks, separated by 0.41$\pm$0.02 in phase, with the first gamma-ray peak
lagging the first radio peak by 0.05$\pm$0.02 in phase, consistent with the
phase of giant radio pulses. We observe significant emission in the off-peak
interval of PSR B1821-24 with a best-fit LAT position inconsistent with the
core of M28. We do not detect significant gamma-ray pulsations at the spin or
orbital periods from any other known pulsar in M28, and we place limits on the
number of energetic pulsars in the cluster. The derived gamma-ray efficiency,
~2%, is typical of other gamma-ray pulsars with comparable spin-down power,
suggesting that the measured spin-down rate ($2.2\times10^{36}$ erg s$^{-1}$)
is not appreciably distorted by acceleration in the cluster potential. This
confirms PSR B1821-24 as the second very energetic millisecond pulsar in a
globular cluster and raises the question of whether these represent a separate
class of objects that only form in regions of very high stellar density
