To "settle on" something means to finally make a decision after considering lots of different choices. You use "settle on ___" to talk about the topic you're deciding:

Have you settled on where you're going for your honeymoon?

Or you can also use it to talk about which one you chose:

We settled on Bali for our honeymoon.

In the U.S., it's common for people to make a general plan to meet people on a certain day or weekend, and then to choose the specifics later.

"Settle on ___" is OK to use in written or spoken English.