Every once in a while, on a bank holiday or a national holiday like MLK Day or Columbus Day, someone asks the question on Writer.net - Are the agents in the office today?
The answer is: Yes. No. Maybe.
An agent's schedule is an increasingly nebulous thing. Certainly, almost nobody works the traditional hours of 9-5pm, Monday through Friday. There's no particular reason to do that. Queries can be read at any time, email means agents can reply at night, and a lot of the business isn't in the office anyway. It's on the cell phone, on a portable laptop, or shuttling from lunch meeting with editor to lunch meeting with publicist to lunch meeting with author. Many agents work out of their home a lot of the time, and have various services where their work phone is redirected to their cell or home phone. Some agents don't have offices at all - or have offices for postal purposes, but they don't really exist. Downtown offices are expensive (easily $1400 a month), but many corporations offer services like providing a mailbox and a phone forwarding service and use of a conference room, so it appears your letter is going to a traditional office setting, when instead it's just being picked up by the agent and brought back to Brooklyn.
The only reason to keep a somewhat normal schedule at all (besides not being a vampire) is because publishing houses do "officially" keep more normal hours. That is, unless the agent knows the editor's private line number or his/her home number. Which the agent probably does.
My first boss used to come in at 11am after working at home for an hour or so. She would check her messages, make a couple important calls, and then leave at 12:30 for a three hour lunch with an editor, which was really little more than a business meeting with food involved and a bill at the end. She would show up again around 3pm, then still be working when I went home at 7.
My current boss has a kid, so sometimes she'll leave if school gets called off (for a snow day or something) and finish her work at home. Other times she'll come in early, go to yoga for two hours in the middle of the day, and then work until about 6.
What I'm trying to say is, you have no way of knowing when we're in the office or not, but it's not really all that relevant. The only time work fully stops is usually Sunday, or a major holiday like Christmas, when even if the agent is available, nobody else in publishing is and the mail doesn't come. The reason we're not answering our phone is not because we're not there. It's because we screen all our calls because most of them will be people pitching their novel on the answering machine, and if we picked up, we might have to listen to them. Send a query.