Aside from race weekends, it's very rare to see the current generation of Formula One cars out on track. This is because teams are restricted in the number of times they can run their latest creations.
The test events and the number of days is the same for every team, and each is allowed two promotional events (filming days capped to a maximum distance of 100km each) and two demonstration events (with a maximum distance of 15km) per season.
This season, there have been two in-season tests, where teams can run one car each. This car is often a different chassis to those that race every other weekend, but sometimes the race cars are used at the tests. The engine and gearboxes are test items, though, and don't come from the pool for the season.
The first in-season test was after the Bahrain Grand Prix, with two days of running in the Sakhir desert, and the second is taking place on the Tuesday and Wednesday after the Spanish Grand Prix.