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]