How do fairies celebrate an anniversary? By flying around the W Boston, of course. The hotel and its restaurant Market hosted a “Midsummer Night’s Masquerade” Thursday night to celebrate the third anniversary of the American Repertory Theater’s run of “The Donkey Show” (which is based on “A Midsummer Night’s Dream”) and to raise money for ART programs. Guests included “Donkey” cast members, radio personality 104.1’s Fast Freddy, “The Accidental Billionaires” writer Ben Mezrich and his wife, Tonya, and Kiss 108’s Jim Clerkin.