From Activision blog(blog.activision.com)
Ready for some weekend warfare? Starting at 10AM PT on April 3, free-to-play Warzone players can jump into Call of Duty: Modern Warfare Multiplayer for free. That’s right: all Modern Warfare and free-to-play Warzone players can square up together in a varied and fast-paced Multiplayer Moshpit, all weekend long.
If you haven’t played Modern Warfare Multiplayer, now’s the chance to see what you’ve missed. Experience the world-class gameplay, deep weapon customization, and gritty, photorealistic environments that make Modern Warfare Multiplayer the ultimate online playground.