This is the 13572nd most frequent Russian word.
начаться
"To begin" or "to start."
Here, 'начинается' is used to indicate that the action of starting applies to the subject (day) in general terms.
Мой день часто начинается с чашки кофе.
My day often begins with a cup of coffee.
In this sentence, 'начался' is used to express that the performance initiated or commenced at a specific time in the past.
Спектакль начался вовремя, как было запланировано.
The performance started on time, as planned.
This sentence uses 'начнётся' to describe a scheduled action expected to begin in the future.