Source : Webster's Revised Unabridged Dictionary (1913)

Happen \Hap"pen\, v. i. [imp. & p. p. {Happened}; 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.

            There shall no evil happen to the just. --Prov. xii.
                                                  21.

   2. To take place; to occur.

            All these things which had happened.  --Luke xxiv.
                                                  14.

   {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.]

Source : WordNet®

happening
     adj : taking place; "the parade is still happening"
     n : an event that happens [syn: {occurrence}, {natural event}]