Rahu kaal is a period of about ninety minutes each day (one eighth of the daylight) governed by Rahu. Tradition considers it a poor time to begin something new, because the energy of Rahu is associated with confusion, delay and things that do not go as planned.
How it is calculated
Take the time from sunrise to sunset and divide it into eight equal parts. Rahu kaal is a different part on each weekday: the 8th part on Sunday, the 2nd on Monday, the 7th on Tuesday, the 5th on Wednesday, the 6th on Thursday, the 4th on Friday and the 3rd on Saturday.
Because daylight is longer in summer and shorter in winter, and because sunrise differs by city, the clock time of Rahu kaal changes daily and varies by place. A printed table made for one city will not be right in another, which is why we calculate it for yours.
What is traditionally avoided
Starting a new business, buying property or a vehicle, signing important agreements, beginning a journey, and starting auspicious ceremonies. Routine work, ongoing journeys and everyday tasks are not affected. If a task has to be done, many people simply begin just before or just after the window.
Yamaganda and Gulika kaal
Two more eighths of the day are watched in the same way. Yamaganda is thought unsuitable for new beginnings, much like Rahu kaal. Gulika kaal is considered less harmful and some traditions even call it good for things you hope to repeat.
What to use instead
Abhijit muhurat, the window around local noon, is considered auspicious for almost any start. The choghadiya of the day also marks good (Amrit, Shubh, Labh) and poor (Udveg, Rog, Kaal) windows.