There can only be One Day astronomically, when the Sun’s Gross RA, HIs Angle of Declination & His Longitude = Zero Degree. That Day is Vishu. That Day is Tomorrow. Scriptural Sanction ? Yes ! Yadha Mesha Tulayo Vartate Thadha Ahoratranam Samanani Bhavanthi ! Day Duration = Night Duration = 12 hrs on that day.