Finally got this done after a while of working on it!

So long story short, Tommy undergoes a couple transformations throughout the year, depending on the season.

His Deer form occurs on any full moon after the Autumnal equinox, which is usually about September 23rd, until the Winter solstice, which is about December 21st or 22nd.

His Snow Leopard form occurs on any full moon after the Winter Solstice until the Vernal [Spring] Equinox, which is usually March 21st.

His Jackalope form occurs on any full moon after the Vernal Equinox until the Summer Solstice, which is about June 20th or 21st.

His Shark form occurs on any full moon after the Summer Solstice, until the Autumnal Equinox, which as I said before it's around September 23rd.

He does revert back to normal though on New Moons. So in short he shifts for about 15ish days then reverts back to normal.