WordPress SEO - Register current Month custom tag or variable in to the Yoast SEO plugin. We can use the %%current_month%% into the title. . Read more at https://maheshwaghmare.com/doc/yoast-seo-custom-template-variable/

add-current-month-tag.php

<?php
if( ! function_exists( 'prefix_yoast_register_current_month' ) ) :
	/**
	 * Register current Month variable in Yoast SEO plugin.
	 * 
	 * @todo Change the `prefix_` with your own custom prefix.
	 * @since 1.0.0
	 * @return void
	 */
	function prefix_yoast_register_current_month() {
	    wpseo_register_var_replacement( '%%current_month%%', 'prefix_get_current_month', 'advanced', 'Current Month..' );
	}
	add_action('wpseo_register_extra_replacements', 'prefix_yoast_register_current_month');
endif;

if( ! function_exists( 'prefix_get_current_month' ) ) :
	/**
	 * Return current Month
	 *
	 * @todo Change the `prefix_` with your own custom prefix.
	 * @since 1.0.0
	 * @return string
	 */
	function prefix_get_current_month() {
		return date("F");
	}
endif;