According to the anime schedule list on Moon Phase, Sasameki Koto is going to air this coming fall. So...I'm guessing it will begin in October, since that's when basically all of the fall titles premiere each year.
I'm massively excited about having two consecutive seasons of anime adaptations of great yuri manga!! ^^ (Aoi Hana in the summer and Sasameki Koto in the fall.)
There hasn't been any indication as to how long either series will be. They could both easily run for 26 episodes, but perhaps one or both of them will get an approximately 13 episode season, and then receive a second season if the first one is popular enough, a la Marimite. We'll just have to wait and see.