The Collaborative International Dictionary of English v.0.48:

Happen \Hap"pen\ (h[a^]p"p'n), v. i. [imp. & p. p. Happened
   (-p'nd); p. pr. & vb. n. Happening.] [OE. happenen, hapnen.
   See Hap to happen.]
   1. To come by chance; to come without previous expectation;
      to fall out.
      [1913 Webster]

            There shall no evil happen to the just. --Prov. xii.
                                                  21.
      [1913 Webster]

   2. To take place; to occur.
      [1913 Webster]

            All these things which had happened.  --Luke xxiv.
                                                  14.
      [1913 Webster]

   To happen on, to meet with; to fall or light upon. "I have
      happened on some other accounts." --Graunt.

   To happen in, to make a casual call. [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

happening \happening\ n.
   1. something that happens; an occurrence; an event.

   Syn: occurrence, natural event.
        [WordNet 1.5]

   2. Specifically: An event that is particularly interesting,
      noteworthy, or important.
      [PJC]

   3. An artistic or entertainment event that is unconventional,
      sometimes discontinuous, designed to evoke strong
      emotions, and sometimes involving participation by the
      audience.
      [PJC]

WordNet (r) 3.0 (2006):

happening
    n 1: an event that happens [syn: happening, occurrence,
         occurrent, natural event]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "happening":
   Grand Guignol, Passion play, Tom show, accident, accidental,
   accompanying, ado, adventure, afloat, afoot, antimasque,
   audience success, ballet, bomb, broadcast drama, burlesque show,
   casualty, chance, chance hit, charade, circumstance,
   circumstantial, cliff hanger, closet drama, comedy drama,
   contingency, contingent, critical success, current, daytime serial,
   dialogue, documentary drama, doing, drama, dramalogue,
   dramatic play, dramatic series, duodrama, duologue, epic theater,
   episode, event, eventuating, experience, experimental theater,
   extravaganza, fact, failure, flop, fluke, fortuity, freak accident,
   gasser, giveaway, go, going on, hap, happenstance, hazard, hit,
   hit show, improvisational drama, in hand, in the wind, incident,
   incidental, legitimate drama, long odds, long shot, lucky shot,
   masque, matter of fact, melodrama, minstrel show, miracle,
   miracle play, monodrama, monologue, morality, morality play,
   music drama, musical revue, mystery, mystery play, occasion,
   occasional, occurrence, occurring, on, on foot, ongoing, opera,
   pageant, panel show, pantomime, particular, passing, pastoral,
   pastoral drama, phenomenon, piece, play, playlet, prevailing,
   prevalent, problem play, psychodrama, quiz show, radio drama,
   reality, resultant, review, revue, sensational play, serial, show,
   sitcom, situation comedy, sketch, skit, soap, soap opera,
   sociodrama, spectacle, stage play, stage show, straight drama,
   success, suspense drama, tableau, tableau vivant, taking place,
   talk show, teleplay, television drama, television play,
   theater of cruelty, thing, total theater, turn of events,
   under way, variety show, vaudeville, vaudeville show, vehicle,
   word-of-mouth success, work