By default, WordPress.com blogs display posts in reverse chronological order on the home page with the latest post at the top. There isn’t a way to display posts in chronological order, but you can mark some posts as Sticky to make them appear above the other posts. They will stay at the top of your posts until you uncheck that option on the Edit screen for that post.
You can mark a post as sticky by checking the option that says “Stick this post to the front page” in the Publish settings on any post.
Sticky posts appear highlighted in some way, depending on the theme you’re using. Usually Sticky posts display a colored background behind the title.
Generally, the Sticky option is only used for one or two posts on a blog. It should not be used for every post. Be sure to uncheck the Sticky option on that post to move it back to its place in chronological order.