ṣukla paksha ekādasi, the 11th day in the bright half of the moon cycle (before full moon), falls during dhanur solar month in Hindu calendar. dhanur month or dhanurmasa is also known as mārgazhi. Vaikuntha Ekadasi is ṣukla paksha ekādasi. Vaikuntha Ekadasi is observed according to solar calendar and because of that it might fall either in margashirsha or in pausha/pushya month in Hindu Lunar calendar. In the Gregorian calendar there may be none, one or two Vaikuntha Ekadasis in a year.
Benefits - Vaikunta Ekadasi is also known as mukkoti ekādasi. It is believed that the Vaikuntha Dwaram or the gate of Lord's inner sanctum is opened on this day. Devotees who observe fast on Vaikuntha Ekadasi attain salvation by going to Heaven. Also to note, Nammazhwar is said to have attained Moksha on this day.