For roughly two weeks in mid-to-late February, sunset light can turn Horsetail Fall into a glowing orange ribbon on El Capitan. Three conditions must hold at once: water in the fall from recent rain or snowmelt, a clear western horizon at sunset, and the February sun angle. The glow builds for a few minutes, peaks near sunset, and is finished about ten minutes later.
The classic viewing zone is Northside Drive near the El Capitan Picnic Area; parking rules and any reservation requirement change annually, so check the NPS Horsetail Fall page before committing.
The full guide: the complete firefall article. Live water and weather: the conditions page.