On your Xbox One, search for City: Skylines. It will be available to install. I can't find any info on how long this will last. Typically free play days are for the weekend, so likely it will stop working after Sunday. If I find a webpage with the exact dates/times I'll update the post.
Link for reference only. You need to install it via your Xbox One console.
It is also on sale for $29.99 for a limited time.