The 49-year period of seven sabbatical cycles, followed by the Jubilee year, parallels the annual 49-day period of seven weeks during which we are to count the days from Passover until the holiday of Shavu'ot, the annual reliving of the Giving of the Torah.
In the annual count from Passover to Shavu'ot, the final, 50th day is observed but not counted. This is because the annual reliving of the Giving of the Torah is a Divine revelation that we cannot attain on our own; it is a Divine gift. Similarly, the Jubilee year is observed but not counted, for the same reason.
Nonetheless, the Divine revelations on both Shavu'ot and in the Jubilee year occur only in response to our having counted the preceding 49 days or years, respectively, through which we ascend through all the levels of Divine consciousness that we can attain on our own.
Thus, we see that with regard to fulfilling our Divine mission, G-d grants us success beyond whatever we can accomplish ourselves -- provided that we invest our maximum efforts to accomplish what we can on our own.
--From Kehot's Chumash Vayikra