Comments on all pages and posts on every WordPress blog are enabled by default. This page > Settings > Discussion
Discussion settings is where you set up defaults for comments.
http://en.support.wordpress.com/settings/discussion-settings/

We can also override the default settings and enable or disable future comments on any individual post or page.
http://en.support.wordpress.com/enable-disable-comments/

Note: By default, new blogs on WordPress.com will have the Discussion hidden. You can reveal it by clicking on Screen Options http://en.support.wordpress.com/screen-options/ on the top right corner of your Admin page, and checking the box that says Discussion so the module is included. http://en.support.wordpress.com/modules/#list-of-modules

Depending on the theme we use when we close comments a phrase like “comments closed” or a similar phrase may appear and if so, it can only be removed by CSS editing after purchasing a custom design upgrade.
http://en.support.wordpress.com/custom-design/
http://en.support.wordpress.com/editing-css/

Thank you!
Blog url: http://workinmotion.wordpress.com/