WordPress/wp-content/themes/twentythirteen/category.php

<?php
/**
 * The template for displaying Category pages
 *
 * @link https://developer.wordpress.org/themes/basics/template-hierarchy/
 *
 * @package WordPress
 * @subpackage Twenty_Thirteen
 * @since Twenty Thirteen 1.0
 */

get_header(); ?>

	<div id="primary" class="content-area">
		<div id="content" class="site-content" role="main">

		<?php if ( have_posts() ) : ?>
			<header class="archive-header">
				<h1 class="archive-title">
				<?php
				/* translators: %s: Category title. */
				printf( __( 'Category Archives: %s', 'twentythirteen' ), single_cat_title( '', false ) );
				?>
				</h1>

				<?php if ( category_description() ) : // Show an optional category description. ?>
				<div class="archive-meta"><?php echo category_description(); ?></div>
				<?php endif; ?>
			</header><!-- .archive-header -->

			<?php
			// Start the loop.
			while ( have_posts() ) :
				the_post();
				?>
				<?php get_template_part( 'content', get_post_format() ); ?>
			<?php endwhile; ?>

			<?php twentythirteen_paging_nav(); ?>

		<?php else : ?>
			<?php get_template_part( 'content', 'none' ); ?>
		<?php endif; ?>

		</div><!-- #content -->
	</div><!-- #primary -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>