Categories

I suspect you have set in Options > Reading to show 1 post on the a webpage. That applies to all your blog pages, be it monthly or category archives.
That’s how WP work, the only workaround is using plugins. Here are two of them:
https://mattread.com/archives/2005/03/custom-query-string-plugin/
https://rephrase.net/miscellany/05/perpage.phps

No what I want is it to display all the articles in that category/month, but only the title and an excert.

You will have to edit your archive.php template and replace the_content with the the_excerpt.
That will display only the excerpt and not the whole article.
However, because you set your Reading Options to 1 (one) post – it will display 1 (one) excerpt. Use the plugins I linked.

Hi what I wanted to I think, and I am sorry for the confusion, is to be able to have a list of all the articles in that month. Like I have for the recent articles list, I want to be able to click say December and every article written in December will be in a list form, which I can then link to view the article.