  We present observations of the gamma-ray blazar Markarian 501 between April
17 and May 5, 2009, with the Whipple 10-m telescope, VERITAS, the Fermi Large
Area Telescope (LAT), Swift and RXTE as part of a 4.5-month multi-wavelength
campaign. The presentation will focus on the strong very high energy (VHE)
gamma-ray activity detected on May 1st with Whipple and VERITAS, when the
measured flux (E > 400 GeV) reached five times the flux of the Crab Nebula,
coincident with an increase in the optical polarization by a factor of 5, and a
rotation of the polarization angle by 15 degrees. We also show that, during
this 3-week period, the largest flux and spectral variability is seen at the
highest energies of the broadband spectral energy distribution.
