Singh Sankranti/Walke Parwa

The solar transition from one zodiac sign to another is called Sakranti, simply let's consider this as the first date of a lunar month. Simha Sakranti is one among 12 Sankranti's of a year, Simha is a transition of the sun from Karka (Cancer) to Leo (Simha). So the destination zodiac gives it a name, Simha Sakranti.