I just watched a National Geographic documentary on ExoMars. In it, they mentioned there were 2 launch windows in early 2016 for the TGO/Schiaparelli launch, one in January and one in March.
Quote from ESA site showing this is not just a gaffe by NatGeo:
The later window is open 14-25 March and, thanks to the relative orbital positions of Earth and Mars, the mission will still arrive at Mars in October, just as if launched in January.
How does that work? As I understand it, there is one optimal date every 780 days, with the required launch energy going up slowly either side of the optimal date. So where do two launch windows, 2 months apart, come from?