SpaceX has admitted that the first Red Dragon - an unmanned mission to Mars - will not be ready in time for the 2018 launch window. Instead, they plan on sending two of the craft in the 2020 window (already rather crowded). It's most efficient to send missions to Mars during the regular "window," which happens every 26 months, when the two planets are closest together.
This isn't officially announced yet, but it's looking very likely. In the same window, NASA will be launching a new lander.
I'm honestly looking more at the SpaceX missions, though, because Red Dragon is designed to easily scale up to a manned mission and to test human-rated descent and landing technology (which is why it's being delayed - they want to get it right. When you can only test something every 26 months...)