One of the most difficult things to learn as a manager is how to own your calendar. It is amazing how many people invite you to meetings or want your opinion simply because of your title.
Many times, at least when you first start managing, you say "yes" to most of the meetings. Partly because you don't know what the meetings are and think if someone invited you, it must be important. Partly because you think that is what a manager does, sit in meetings.
The reality is that you will probably attend more meetings than you did before. But unless you are judicious about which meetings you attend, it will take up too much of your time. Here are the things you can do to ensure you own your calendar.
Do not accept a meeting until you are sure of its purpose and why you need to be there. I know some managers who set their calendar to automatically accept every meeting - DON'T DO THAT. If there isn't a good reason for you to attend, say maybe or no. If you don't know what purpose you have in attending, ask the meeting organizer.
Do not double-book times. Some managers strangely wear it as a badge of honor ("I'm double booked most of the day!"). It doesn't mean you are valuable, it means you can't make decisions.
Evaluate your task list. If you have 8 hours worth of tasks that HAVE to get done, scheduling 5 hours of meetings seems like a really bad idea. You have to make hard decisions.
Be honest with meeting creators. If you won't be there, tell them that and tell them why. This gives them the ability to either go ahead without you or reschedule if you really need to be there. The sooner the better. Telling the meeting organizer 10 minutes before doesn't help.
Send someone on your team in your place. Make sure they are prepared for the meeting and let the organizer know that they are there and can speak for you. This frees you up and can be a development opportunity for the employee. At worst, they get exposure to new people to add to their network.
If you happen to have an assistant who controls your calendar, that doesn't mean you are off the hook. You should communicate with the assistant at least once a day to review the calendar and give them the information they need to make smart decisions. Your assistant is your responsibility - so if they make the wrong decision, it is your fault.