The dates for the 10-day passes vary, according to Notorious' latest press release. If you're a previous subscriber, check your email over the next few days to see where your game time falls within the larger five-week promotional window.
Also noteworthy for dedicated Xsyon players is the fact that the game's totem system will enter something called a decay phase once these trial periods are over. Totems represent land claimed by a particular guild, and those without any actively subscribed players will become abandoned in short order. Abandoned totems will then decay and free up the land for other players, while also leaving any buildings, resources, and containers free for the taking.
[Source: Notorious Games press release]