Vishnu Sahasranama Jayanti is the most auspicious day during when Vishnu Sahasranamam was revealed by Bhishma to Pandavas. In fact Bhishma pitamaha was united with the Lord on Bhishmashtami day. Before his last breath, Bhishma prayed to the Lord with 1000 names of Vishnu and those names have become famous after his death.
But, this Magh Shukla Ekadasi has become popular as Vishnu Sahasranama Jayanti. Vishnu Sahastanama Jayanti 2020 date is February 5.
Vishnu Sahasranama stotram is the most chanted or recited stotram dedicated Lord Vishnu. This prayer of 1000 names of the Lord is chanted on all Ekadasi vratas.