These two phrases are very similar and can often be used interchangeably. The "office will close" might mean that an office will close at a specific time, while the "office will be closed" does not indicate this as strongly.

The phrase “will be closed” usually indicates that a place or business is not open for a certain amount of time and that it will not reopen until a specified date or time. For example, Route 40 will be closed until 7:30 tomorrow night. This means that the route will not open again until 7:30 tomorrow night.

Closed (/kləʊzd/) can be the past tense form of the verb close, but it can also function as an adjective. As an adjective, closed means 'not open'. Confusion often arises when people intend to describe something as not open, which requires the use of the adjective closed, but they mistakenly use the word close instead.