What is the Vishnu Sahasranaamam? Why is it important? During what times of day can one recite it? What are the important verses in it? What is the history of it?
The word Sahasranamam is to be split-ed as Sahasra + Namam. Sahasra means "one thousand" and Namam means "names" in Sanskrit. Thus, the term Vishnu Sahasranamam gets us the meaning 1000 names of Vishnu.
In Hinduism, Naama worship (names worship, also said as நாம சங்கீர்த்தனை nāma caṅkīrttaṉai in Tamil) is considered as much more powerful than other forms of sloka worship. There are no restrictions for Vishnu Sahasranamam worship in a day. Literally, one can worship this verses right after getting up even before bathing.
The most important verse in this worship is as follows,
Sri Raama Raama Raamaethi Rame Raamey Manoramey
Sahasranaama Dhathulyam Raama Naama varaananey
History: Yudhistira was uncertain about which is the biggest Dharma to follow in life. Lord Krishna took him to the battlefield where Bhishmachaarya was lying in a bed of arrows, during his last minutes. Bhishmachaarya was very ill and could not answer all the questions. At the time he meditated on Lord Krishna, and Lord Krishna offered his blessings to Bhishmachaarya. After this, Bhishmachaarya answered Yudhistira's question.
Yudhistira's question: Kimaegam thai vatham loakae kim vaapyaegam paraayanam sthuvanhtha gam gamarchchandha praapnuyar vaanavasa subam.
Meaning: Who is the greatest God in the world? Who is the one saves all? By worshipping whom, one can become auspicious? What is the greatest Dharma according to you? By worshipping what, one can get rid of the bonds of this world?
Bhishmaachaarya answered Yudhistira as, whoever brought you to this world is the greatest Lord and he is the one to whom anyone must surrender. He advised meditating on his 1000 names, by means of which one can reach the auspicious state and get rid of bonds and Karmas. He then recited the 1000 names of Lord Vishnu. This part of Mahabharatha called "Vishnu Sahasranamam".