Translation:Mother's day will be next month.
Absolutely right. In English we say, "Mother's Day (or any other event) IS next month". My guess is that this is the norm for impending (as in next week, month, semester) events, because when speaking of events further out, we would use the explicit future, e.g. "Christmas will be on a Tuesday next year."
Native Russian speaker here: this is a perfectly normal sentence for both of the meanings. If nitpicking, you could leave off 'будет' and it would be a little bit better for implying what you are talking about, but the difference is almost nonexistent - it would still have both meanings