početkom

1. (+ gen.) at the beginning of; early in
   Selimo se početkom sledećeg meseca. — We're moving at the beginning of next month.
   Početkom devedesetih živeli su u Beogradu. — In the early nineties they lived in Belgrade.

Instrumental of početak, used as a preposition with the genitive. Sredinom and krajem work the same way: sredinom maja, krajem godine.