Why the sky goes red
At sunrise and sunset, sunlight travels through far more atmosphere than at midday. The blue light scatters away, leaving reds and oranges — and the sky only glows that deep red when the air is dry and dusty enough to scatter the light strongly.
A red sky therefore tells you something real: the air in that direction is dry and relatively clear.
Why the rhyme works
In mid-latitudes like Britain and most of North America, weather systems travel west to east. A red sunset means dry, clear air to the west — which is where tomorrow's weather is coming from. Hence: delight.
A red sunrise means the clear air has already passed east, and the sun is lighting the underside of whatever is arriving from the west. Hence: warning.
When it fails
The rhyme assumes weather moves west to east, which is often true and often not. Tropics, local sea breezes, stalled fronts and stubborn summer patterns all break the rule. It is a tendency, not a law.
Treat it the way the shepherds did: as one voice in the conversation, not the final word.
Reading your event-eve sky
If the sky the night before your outdoor event burns deep red over a clear western horizon, folklore and physics both lean your way. It is the single most cheering thing you can see at sunset.
Take a photograph of it. If the next day is fine, you will have the pair of pictures forever — the omen and the day it promised.
The witch's verdict
This is the piece of weather lore we would keep if we could keep only one. It is beautiful, it is ancient, and it is right often enough to have earned its thousand years.
The rest of the craft — the candles, the knots, the petitions — is for everything the red sky cannot tell you.