'Until' usually doesn't indicate whether or not it includes the designated time. The 'returning 24/6/2012' part is added to clarify this, indicating he will work on that day.
By the way, your question should be: "On which day will he come?/Which day will he come on?" or better yet: "When will he come?".
"I'd like to ask a question. I'm not sure which day this sentence means he will come."