It’s running until the 17th. They also put out a bunch of Hatterenes and other decent support pokemon for the raid. It seems you need at least 3 mews though. If you enter the raid with a Mew, it gets a crazy stat boost and you get some extra stuff when you beat it. The loot for beating it multiple times isn’t as good just FYI.
Even if you don’t do the Mewtwo raid, now is a good time to farm Blissey raid battles because they drop insane amounts of XP candy. A solid hour of farming them can net you hundreds of L candies pretty easily. I’ve been using Belly Drum/Drain Punch Iron Hands for that.
I know it’s not free, but I think you can subscribe for a month for $4USD and that would give you 30 days. The raid will be over before the subscription ends.