Ekadashi is the eleventh day, falling both under the Shukla Paksh as well as Krishna Paksh. There are two Ekadashis in a month and hence, twenty four every year. When there is an extra month in a year based on the Hindu calender, that month is known as the Mal Maha/Adhik Maha/Purushottam Maha. 'Maha' which means month in Sanskrit and Hindi, is also known as Masa.
Every Ekadashi is associated with Lord Vishnu. However, Lord Shiva is also worshiped on Padmini Ekadashi. The Padmini Ekadashi falls on the eleventh day of Shukla Paksh in the Adhik Maha. Every Ekadashi is considered very auspicious for making donations. There is also a provision to observe a fast on this day. It is said that if a person observes fast on this day, he is blessed with all the benefits that a man gets on performing holy yagnas, charity, other fasts, penance and other many holy rituals. Here is the story for Padmini Ekadashi Vrat.
There was a king named Kirtaveerya. The king had many queens, but they had no child. They were not happy despite having all the pleasures of life. This was the only thought that kept them unhappy all the time. Once, the king along with all his queens, sat on a penance. However, this brought no results and the penance went in vain.
One of the queens then worshiped Goddess Anusuya. The Goddess appeared before the queen and asked her wish, the queen told her the reason of unhappiness of all the queens and the king. The Goddess told the queen that she must observe the Ekadashi fast falling in the Mal month, and worship Lord Vishnu. And the Lord would listen to her prayers for sure.
The queen then asked the Goddess, the procedure for Ekadashi fast. The Goddess gave her the procedure and then disappeared. The queen had utmost faith in the Goddess. Therefore, she followed her orders and observed the Ekadashi fast falling in the Mal month.
Pleased by her worship, God appeared before the queen and told her to ask for a boon or wish. The queen requested God to fulfill the wish of her husband instead of hers. God agreed and told the king to ask for a blessing. The king, surprised to see God, asked him to give a son who possesses all the best qualities, whom no one can defeat other than the God himself and who is respected in all the three worlds, the Aakash, the Patal and the Prithvi Loka. God just said 'tathastu' and disappeared. Tathastu means, 'may it so happen' and after a period of time the queen gave birth to a son, who was named Kartaveerya Arjun.
Kartaveerya Arjun, later, became one of the most popular and famous kings of those times, who is also credited to have captured Ravana once.
The devotes should wake up during the Brahma Muhurta and take bath. Then Lord Vishnu is to be worshiped. The fast is either nirjala, or sajala. Nirjala means without water and no water is taken by the devotees for the whole day. Sajala is the one done with water. Actually nothing has to be eaten the whole day, but many also eat fruits once during the day. The best way of fasting is the Nirjala Vrat. Many also say that having fruits during this fast is not the correct way.
People observe a whole night vigil and ask for the blessings of God.
Both Lord Vishnu and Shiva are to be worshiped. Offer different things to the deity every three hours.
The observer of the fast is supposed to first offer prayers to the deities the next morning, then offer food and Dakshina to a Brahmin, and only then can he have food himself.
This way the person who observes the Ekadashi fast achieves salvation, attains God and gets various other blessings and wishes fulfilled by dear Lord.