In some years, Vishu is held on April 15 and not on April 14. Why? This is because Vishu is decided based on the transition of the sun from Meena Rashi to Mesha Rashi (Meda Rasi). Vishukani and Vishu is observed after the Sun enters Mesha Rashi. Vishu in 2014 is on April 15.
This means the transition should take place before sunrise on April 14 to observe it on the day. If the transition is after sunrise on April 14 then Vishu is observed on April 15.
Thus Medam 1 in Malayalam calendar will be on April 14 but Vishu will be observed the next day on April 15.