Durga Puja starts on 10 October, 2013 evening (Maha Sashti) and ends on 14 October, 2013 (Bijoya Dashami) with the immersion of the idol. The Puja pandals will be crowded from at least 2 days before the actual start of the ceremony.
Railway reservation starts, I am told, 60 days before the journey (please cross check.
In case you hve not seen the Puja earlier in Kolkata, it will be a memorable experience.