Rahu Kaal is a period of roughly ninety minutes each day considered unsuitable for beginning anything new. Unlike most muhurat calculations it is simple arithmetic, which is why it appears on every panchang and why so many people use it.
How it is calculated
Take the time from sunrise to sunset and divide it into eight equal parts. Which part is Rahu Kaal depends only on the weekday, in a fixed order: the eighth part on Sunday, the second on Monday, the seventh on Tuesday, the fifth on Wednesday, the sixth on Thursday, the fourth on Friday and the third on Saturday. The first part is never Rahu Kaal on any day.
Because the calculation starts at sunrise, Rahu Kaal moves through the year and differs by location. A table printed for Delhi is wrong for Chennai by a meaningful margin. This is why a panchang that does not ask where you are should be treated with suspicion.
What it applies to
The tradition applies Rahu Kaal to beginnings: signing, launching, travelling for the first time on a route, buying something significant, starting a course of treatment. It is not applied to work already in progress, and stopping mid-task to wait it out has never been the practice.
What it does not apply to
- Anything already under way — Rahu Kaal governs starts, not continuations.
- Emergencies and medical treatment, without exception.
- Daily routine work, which is not a beginning in the intended sense.
- Marriage muhurats fixed by a full calculation, which override it.
The related periods
Yamaganda and Gulika Kaal are computed the same way, with different weekday orders, and are treated as lesser versions of the same idea. Abhijit muhurat is the opposite: the eighth muhurat of the day, around solar noon, considered favourable for almost anything and able to override several ordinary objections.
Rahu Kaal is worth ninety minutes of patience for a genuine beginning, and no more than that.