To find out when a Gathering will take place, ask a Clan member, or check the Moon Chart.
The Moon chart is located in the navagation bar, beside the home button (little kitty holding a sign reading "HOME!"). Click on the Moon Chart words or picture, and a pop-up window will come on your screan. There will be a chat, with a picture of a moon, reading "Current Moon". It will tell you how full the moon is, plus the date.
Remember, on the half-moon, the medicine cats will meet at their place, aswell.