The Cub Scouting ranks effectively change with the end of the current school year's grade, and work toward the next rank can begin then. Boys are automatically advanced to the next level in the BSA National database on June 1st. However, some Packs go "dark" in the summer and so September is often the actual start toward the next rank.
So, a Tiger finishing first grade begins working on Wolf requirements in June of that year, and has the whole summer to be a Wolf Cub Scout. Cub Day Camps treat boys who have completed a grade as having advanced to the next level for purposes of grouping.
The Cub Scout Leader Book states that Packs can allow a boy a few extra weeks after the typical school year to finish up a rank if he is not yet finished ("few" being undefined and left up to the unit to be decided in the best interests of the Scout).
excerpts from: http://netcommish.com/AskAndy61.asp and the Cub-Scout-Talk list