Bengal is the land of Goddes Durga.  Every year Durga Puja is celebrated for 10 days in the Hindu month of Ashwin (October). 
On the final day the idols are immersed in rivers so that mother Goddess can return to her abode in Himalayas. During this time Bengalis from both side of the border (India and Bangladesh) gather on the banks of River Ichhamoti seperating the two Bengals. And visarjan happens by carrying the idols on boats. This is the only day when the borders are open for all.