Rahu Kalam (also known as Rahu Kaal or Rahu Kaalam) is a period of time in Hindu astrology that is considered inauspicious for starting any new activities or undertaking important tasks. It is believed that during this time, the malevolent planet Rahu is governing and its negative energy can have an adverse effect on the outcome of any task.

The duration of Rahu Kalam varies depending on the location and the length of the day. It is calculated by dividing the number of hours between sunrise and sunset by 8 and the resulting time period is considered as Rahu Kalam. For example, if the time between sunrise and sunset is 12 hours, then Rahu Kalam will be 1.5 hours (12 divided by 8).

Rahu Kalam occurs every day of the week, except for Sundays, when it is believed to be absent. The exact timing of Rahu Kalam changes every day based on the position of the sun and the length of the day.

It is advised to avoid starting any new activities during Rahu Kalam and instead focus on completing any pending tasks. However, if one has no other option but to start a new task during this period, then it is recommended to perform some rituals and prayers to appease the negative effects of Rahu.