According to Vedic astrology Rahu is an inauspicious planet. During transition of planets the time under the influence of Rahu should be avoided to do any auspicious work. Doing Puja, Hawan or Yagya (????) to propitiate auspicious planets during this time is interfered by Rahu due to its malefic nature. If one does Puja, Hawan or Yagya during Rahu Kaal then desired results are not achieved. Hence it is important to consider Rahu Kaal before starting any new work. By doing so, possibility of achieving desired results is increased.
However any work related to Rahu gives good results during this period. Hawan, Yagya, etc. to propitiate Rahu can be done during this time. People, especially in South India, give utmost importance to Rahu Kaal. Auspicious activities like marriage rituals, engagement, Graha Pravesh, any purchase of stocks, shares, gold, home, car and starting new business or trade are avoided during this time. Rahu Kaal is considered only for undertaking any new work and already started work can be continued during Rahu Kaal.