We should cut the lemon which has to be offered to goddess Durga only in Rahu kala. We should not cut and keep it ready before Rahu Kala. This pooja can be done on Tuesdays and Fridays. 3 Ripe yellow lemons have to be taken and cut into equal halves. When you cut 3 lemons, you get 6 pieces(6 halves), one is offered to Ganapati(Lord Ganesh) who had been given the boon by Goddess Parvati