As Sri Durga dasan says there are six navarathris for six rithus.However Devi upasakas generally observe Vasantha Navarathri,Aashada Navarathri,Sharan Navarathri and Pushya Navarathri.Aashada Navarathri is otherwise called Vaarahi Navarathri and Pushya Navarathri is called Shyamala NavarathriThere are actually 6 navarathris for each ridhu. We usually celebrate Sharadha navarathri (Sharath ridhu) and even this vasantha navarathri is also very famous one. Since this falls on hot summer days, it is much celebrated with events asssociated with Thepporsavam, Poornabhishekam, etc.
Pranams