It is an auspicious day for Hindus to worship Lord Vishnu aka Venkateshwara. Iskcon temples around the world will have special pujas on this day.
Moon becomes benefic in waxing cycle and malefic in waning cycle. Thus the waxing cycle of Margashirsha month is considered most auspicious for special religious rituals and for starting new projects especially if it is sattvic in nature.
Also read: The spiritual importance of Margashirsha
Every tithi of this waxing cycle is important and Ekadashi is the 11th lunar day of moon’s cycle and it is connected to Lord Vishnu. Vishnu is residing in Vaikuntam aka the Svarga loka (heaven world), so the devotion towards Lord Vishnu on this day is believed to bring a heavenly abode. Even in vishnu temples the sanctum gate will be opened at the time of brahma muhurta (roughly an hour before sunrise) as a sign that the people who worship vishnu this day will reach the holy feet of lord vishnu. It is usually considered very powerful when you are awake the whole night and worship the deity at the time of Brahma Muhurta. Some people even stay at the temple the whole night to make things easier.
Do’s: You can read Matsya purana, Vishnu Sahasranama Stotram, Bhagavad Gita, Vishnu suktam today for added benefits. Also you can install a new fish tank at home, start a new work that benefits others, involve healing others etc.
The underlying reason is that there are many different Vishnus are there according to puranas and Vaishnava siddhanta. The vishnu we normally worship is one among a few since there are many different worlds (lokas) with different dimensions. According to puranas there are 14 worlds and some claim even more than that. The vishnu we worship is the presiding deity of our world including all 14 lokas. But our universe is inside another universe which is presided by another vishnu and it's like, our vishnu was created by another vishnu from a higher verse who was created by another etc. to make us understand the multidimensions and the multiverse. The same way Lord Shiva is explained in Shaiva siddhanta. Your soul takes many different lives in different worlds (lokas) based on its karma and the travel between these different realms are faster than light (just through consciousness/thought process). When you gain enough good karma you take birth at svarga loka where devas like indra and all nine grahas reside. But a soul can once again take birth here on our world if the karma gets stained again or can go to even higher realms based on karma purification. All these processes are explained in vedic texts.
Also read: ShivaShakti and Shiva tattva