From April 18th to May 19th you can enjoy unlimited access to all DCS aircraft, maps, and assets packs for free.
No, you didn't read it wrong. It is true: from April 18th to May 19th, you will be able to try all modules and all maps for free.
Not only that, you will also be able to purchase most of the addons for 50% off. Some modules, such as the F-14 are not part of this promotion but they are still at a discount.