Full moon
From Tardis Wiki, the free Doctor Who reference
Full moon refers to when half the Moon's surface is illuminated by the Sun and can be seen from Earth.
In order to transform into their true form, Lupine Wavelength Haemovariform use the moonlight from a full moon to gain enough energy for the transformation. However, in contrast, a high concentration of moonlight is also fatal. (TV:Tooth and Claw)