This is the 744th most frequent Spanish word.
comienza
"Comienza" means "it begins" or "it starts" in English.
In this sentence, 'comienza' is used to indicate the start or beginning of an event (a class) at a specific time.
La clase comienza a las ocho de la mañana.
The class starts at eight in the morning.
Here, 'comienza' signifies the start of something regular or cyclical, specifically the start of the day.
Cada día comienza con la salida del sol.
Each day begins with the sunrise.
This usage of 'comienza' implies that an activity (a soccer match) starts contingent upon a certain condition being met (the referee's arrival).