Only the full moon. Any bright night. Superstition about the calendar.

The moon does not emit. At full moon the observer sits at zero phase angle: sun, observer, moon in a line, every surface shadow on the disc gone. Opposition surge adds about forty percent more brightness than a geometric model of the same disc. The surge itself is slightly stronger at shorter visible wavelengths, around 410 to 415 nanometres — melanopsin's secondary shoulder, not its 480-nanometre sky-blue peak. A lamp at the same lux does nothing. Intensity was never the trigger. Geometry plus that band is.

Noon sunlight carries orders of magnitude more 415-nanometre photons. LR-1 does not fire then because the receptor is gated: melatonin must sit above a systemic threshold. Night only. Urban LED and fluorescent light raise the ipRGC floor until the moon's five-to-eight-times increment never arrives. Light pollution is not a lifestyle note. It is a switch. The calendar curse is an optical alignment the rest of mammalian photobiology had no reason to look for.