が is used to report news or information recently discovered ("neutral description ga"). So ～が始まる is used when you want to tell a certain event will start now.
は is used to describe a general fact. ～は始まる is used to describe a fact that's related to the beginning of something. The described "fact" has nothing to do with what's happening in front of you.
- セサミストリートが始まります。 Sesami Street starts (now).
- セサミストリートは日曜日の朝9時に始まります。 Sesami Street starts at 9:00 Sunday morning.
- × セサミストリートは始まります。 (Makes no sense; sounds like "× Sesami Street is a thing that starts.")
See: Can someone explain me the use of は and が in this sentence?