This is also called as Vaikunta Chathurdasi.
On this day, Lord Vishnu did puja to Lord Shiva. Hence it is considered a very pious day. Even Adi Sankaracharya, when his mother was on her death bed, did Shiva puja and after that, Vishnu puja.
There is no difference between Shiva and Vishnu. The differences are created by mere humans(us).