What is the first salat of the day?

It depends on how you define the start of the day. If you define according to your work and normal life conditions, then the first salat of the day is the Fajr Salat (at dawn and before the complete sunrise). If you define the start of the day per religion, then the day starts by sunset and the first salat is the Maghrib salat (at sunset and before the full disappearance of sun light).