What is "to happen on"

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.

   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.]