According to Vedic astrology, certain parts of the day, called Rahu Kalam, are inauspicious periods and one should not undertake any auspicious work during these periods – especially any new undertaking.
In Indian Astrology, each day of the week has been assigned its respective ruling planet viz. Monday is ruled by Moon, Tuesday by Mars etc. However, Rahu and Ketu have not been given any specific day as they are treated as nodes. However, Rahu and Ketu are given one and half hours time in a day resulting three hours for Rahu and Ketu in which they can cast their malefic influence since Rahu is a natural malefic.
Rahu Kalam (Rahu Kaal) is an inauspicious period of time for a particular day. In other words, Rahu Kalam is the time during which Rahu seizes the opportunity for influence. During this time one should avoid any auspicious work like starting new business or project, buying expensive items, investments, business transactions, journeys, meetings, interviews etc. It is believed that any thing lost during Rahu Kalam is never found.
Rahu Kaal is one-eighth part of the difference between the Sunset and Sunrise time of a particular place for a specific day of the week. The reason for its 8th part is that in some Indian astrology systems, Rahu is considered to be the 8th planet.
Rahu kalam differs from place to place depending on Sun-rise & Sun-set time. The day duration from Sun-rise to Sun-set, at a particular location, is divided into eight equal parts. There is no Rahu Kalam during the first period of any weekday. On Monday it falls on the 2nd period, Saturday 3rd period, Friday 4th period, Wednesday 5th period, Thursday 6th period, Tuesday 7th period & Sunday 8th period.
The following are the details of the Rahukalam (also known as rahukal or rahu kal) periods in in operation during the various week days. This is based on 6:00 AM local Sun-rise time and 6:00PM local Sun-set time. If the Sun-rise and Sun-set times are different, you can take the Day duration, divide it in to 8 equal parts and apply the above given formula.