Rama Ekadasi is observed during the waning phase of moon in Kartik month (October – November). In 2015, the date of Rama Ekadashi is November 7. The importance of this Ekadasi was narrated to Yudhishtira by Lord Krishna and is recorded in the Brahmavaivarta Purana. Ekadashi is an important day dedicated to Lord Vishnu in a Hindu lunar month and it occurs twice in a month. Devotees fast on the day and it is believed to absolve sins committed.
It is said that once a weak prince observed Rama Ekadasi fast after his marriage. The wife of the prince used to observe the Ekadashi from her childhood and the prince was emulating his wife. But the weak prince could not survive the fast and he died at night.
But his determination to observe the Ekadasi was rewarded and he became the king of temporary kingdom in heaven. The prince realized that with just observing a single Ekadasi he was able to attain this temporary kingdom and by bringing his wife into the kingdom he will be able to make the kingdom permanent as his wife has been observing Ekadasi from childhood.
The wife was only happy to use her piety for the cause of reunion and thus the husband and wife was united.
All the usual rules associated with Ekadasi fasting is observed during Rama Ekadashi. Grains and rice are avoided on the day by many devotees.
Source – http://www.hindu-blog.com/2008/10/rama-ekadashi.html