  The very high energy (VHE; E $>$ 100 GeV) blazar Markarian 501 was observed
between April 17 and May 5 (MJD 54938--54956), 2009, as part of an extensive
multi-wavelength campaign from radio to VHE. Strong VHE $\gamma$-ray activity
was detected on May 1st with Whipple and VERITAS, when the flux (E $>$ 400 GeV)
increased to 10 times the pre-flare baseline flux ($3.9{\times 10^{-11}}~{\rm
ph~cm^{-2}~s^{-1}}$), reaching five times the flux of the Crab Nebula. This
coincided with a decrease in the optical polarization and a rotation of the
polarization angle by 15$^{\circ}$. This VHE flare showed a fast flux variation
with an increase of a factor $\sim$4 in 25 minutes, and a falling time of
$\sim$50 minutes. We present the observations of the quiescent state previous
to the flare and of the high state after the flare, focusing on the flux and
spectral variability from Whipple, VERITAS, Fermi-LAT, RXTE, and Swift combined
with optical and radio data.
