I’ve fixed this via the ACF documentation – http://www.advancedcustomfields.com/resources/query-posts-custom-fields/.
Using the ‘LIKE’ compare value in the meta_query array:
$args = array(
'numberposts' => -1,
'post_type' => 'activities',
'post_status' => 'publish',
'meta_query' => array (
array (
'key' => 'day_of_the_week',
'value' => $title,
'compare' => 'LIKE'
)
) );