This page > Settings > Discussion
Discussion settings is where you set up defaults for comments.
We can also overide the default settings and enable or disable future comments on any post or page.

Enabling comments on posts and pages when they have been previously disabled is not retraoactive. So if you have posts or pages that were were already published, after you enable comments on future posts and pages, you must manually enable commenting on the posts and pages whereon commenting was previously disabled via bulk editing.

Reference link > http://en.support.wordpress.com/enable-disable-comments/

Go to your Edit Posts Screen (Dashboard > Posts)

http://en.support.wordpress.com/posts/edit-posts-screen/
Or go to your Edit Pages Screen (Dashboard > Pages)

http://en.support.wordpress.com/pages/edit-pages-screen/
Select all the posts or pages you want comments enabled on using the checkboxes on the left.
Go to the Bulk Actions list, select Edit, and click the Apply button.
You will see the options that are available for bulk editing.
Select the Comments menu, then “Allow”.
Click “Update”.

When we close comments a phrase like "comments closed" or a similar phrase will appear and it can only be removed by CSS editing after purchasing a custom design upgrade.