Looking on the internet deeply has found these results:

ill be at the theatre is the most popular phrase on the web.

Related Comparison

• go to the opera or be at the opera?
• They are at the cinema. or They are in the cinema?
• be present at the meeting or be present on the meeting?
• please be at the gate or please be near the gate ?
• be good at the English or be good in the English?
• On the theatre or At the theatre?