As per Vishnu Puran, on the day of Ashadh Shukla Ekadashi, which is also known as Harishayani Ekadashi, Lord Vishnu goes to sleep for four months in Ksheer-sagar (ocean of milk). After four months, Lord Vishnu wakes up on Kartik Shukla Ekadashi. That is why Kartik Ekadashi is being celebrated since ancient times, as it is the time of Deva-uthni, which means the time for the awakening of Gods. This Ekadashi is also known as Tulsi Ekadashi. Tulsi Vivah is also arranged on this day. On this day, Tulsi gets married to Shaligram. It is believed that this type of arrangement fetches blessings of good luck. There is a belief that performing marriage ceremony of Tulsi Vivah fetches the equal boons to the devotee that he gets while doing Kanyadaan (donating daughter by getting her married). This year, this Ekadashi is going to be celebrated on 24th of November 2012.
