Hoping for a call from Mars? You've got about a month to wait. Starting today and running through the end of April, Mars and the Earth will be out of sight of each other. In an arrangement called conjunction, Mars and the Sun are lined up with each other, meaning that the much-larger Sun blocks our view of the red planet. This situation occurs every two years and is no big deal - unless you're a Mars spacecraft hoping to communicate with the Earth.
Five spacecraft, three orbiters and two rovers, currently study Mars and they keep up a daily chatter with mission controllers here on Earth. From mundane information like current power levels to critical information such as navigation updates, a stream of data is constantly flowing. For the next month, though, that stream will dry up. This triggers a rest for both Curiosity and Opportunity. Although both rovers have a limited ability to navigate by themselves, it's much safer just to settle in and wait. Curiosity will be studying water in the soil underneath it, while Opportunity will be making dust measurements and examining a rock.
The orbiters, Mars Reconnaissance Orbiter, Mars Odyssey, and Mars Express, are a bit more autonomous. They'll continue carrying out their missions while storing the data they collect. In an attempt to measure how disrupted communications are during this period, Odyssey will continue to attempt contact with Earth and record the results. This could provide useful information when conjunction rolls around in another two years.