Your kitties and pups understand your thoughts & feelings way more than your words.
So, if you are going to be away, or if they are going away (if they're going to be boarded, or stay with a friend), you'll be doing them a good deed if you let them know. In advance. Same applies if you are having visitors in your home for the holidays. Let them know what's going to be happening.
It will greatly help them (and you!) if you tell them in ways they can best understand.
Here's how:
Make a movie and play it out to them, using your mind and imagination. Share images and feelings and whatever would help them "get" it... Project all of it to them. Show them the activity, the people, how they'll be safe... Whatever you would want them to know.
Run it like a little movie that you two are watching together – and you are the narrator and feeling generator. If you are going to be separated from them, you'll want to show them your reuniting with them... your return, your home-coming. If it'll be a while, you could show them how many mornings or nights... Relay whatever you think will help them understand what and why.
Remember, they do not need a lot of words explaining the details. That's not important to them. Just give them what is going to directly affect them - and what you want them to know.
Give it a try. It's a lot easier to step up your animal communication skills than you might think. Play with it... There's no way to get this one "wrong." You just taking the time, with sincere intention and effort, gets you most of the way there.
Here's a little video that may help: