What is Rahu Kaalam and how to calculate it
Rahu kaalam (or Rahu kaal, Rahu kalam, Rahukaalam), is an approx 90-min period during the day that is considered inauspicious to begin any puja (like Ganesh Chaturthi or Satyanarayana Puja) or other undertaking (like a housewarming). It is at different times each day of the week.
First, divide the time between sunrise and sunset in your location into 8 equal segments and then calculate Rahu kaalam as follows (this example assumes 6am sunrise and 6pm sunset):
Monday (2nd segment): 730am-900am
Tuesday (7th segment): 3pm-430pm
Wednesday (5th segment): 12pm-130pm
Thursday (6th segment): 130pm-3pm
Friday (4th segment): 1030am-12pm
Saturday (3rd segment): 9am-1030am
Sunday (8th segment): 430pm-6pm
How did it start
Rahu is in the Atharvaveda as the cause of eclipses and meteors in Hindu astrology (Jyotishya). In Vedic astrology, each day is divided into 8 equal segments between sunrise and sunset, and each segment is said to be under the influence of one of the grahas (planets). Rahu is a graha not affiliated with an actual planet, and is hence a ‘shadow’ planet.
What is its significance today
Some say it denotes Earth’s closeness with various planetary bodies during ‘rahu’ periods of the day, but ultimately your belief in Rahu kaalam runs on your faith in Hindu astrology. It is a helpful placeholder to bring predictability in when you start and finish pujas or other rituals.