Our tradition gives us two seasons of counting. In the (northern hemisphere) spring we count the 49 days between Pesach and Shavuot, also known as the Omer. As we count the Omer, we connect our festival of liberation with our festival of revelation. It's an opportunity to be mindful of the passage of time, and also to do the important spiritual work required each year to get us into the right spiritual and emotional space to receive revelation anew.
And in the (northern hemisphere) late summer / early autumn we count 49 days between Tisha b'Av and Rosh Hashanah. (This is why my friend and colleague Shifrah Tobacman's book of poems is called Omer/Teshuvah -- turn it one way and you can use each poem as a daily meditation for the Omer count; turn it the other way, and you can use each poem as a daily meditation for the journey to Rosh Hashanah.)
Another interpretation: instead of counting the 7 weeks between Tisha b'Av and Rosh Hashanah, some count the 40 days from Rosh Chodesh Elul (the first day of the month of Elul) until Yom Kippur. The number forty has special significance in the rabbinic imagination. Forty were the days of the Flood, forty were the days Moshe spent atop Sinai; forty are the weeks between conception and birth (well, more or less -- that's how the sages of our tradition understood it, anyway) -- so forty can represent the transition from one stage into the next. A kosher mikvah must contain forty measures of water, and as a mikvah purifies the one who immerses in it wholly, so can this time of year purify we who immerse in it wholly.
What's the point of all of the counting? Is it just another opportunity to exercise our uniquely Jewish form of something akin to O.C.D? I don't think so -- or at least I don't think that's all it is. We count the days between one thing and the next because that helps us stay situated in this moment in time. The counting can help us combat the tendency to draft either into the remembered past or into the anticipated future. Beyond that, it links us both with that past and with that future. Today is the eleventh day of Elul. We've been trying to do the work of teshuvah for eleven days: how's that working out for us so far? We have the rest of Elul for our internal work, and then the ten first days of Tishri for external work, repairing our relationships with others and with the world. When we count the days, we keep track of where we are -- how far we've come -- and how far we have yet to go.