function mfp_Add_Text_To_Excerpt($old_Excerpt)

// Hook the get_the_excerpt filter hook, run the function named mfp_Add_Text_To_Excerpt
add_filter("get_the_excerpt", "mfp_Add_Text_To_Excerpt");

// If today is a Thursday, remove the filter from the_excerpt()
if (date("l") === "Thursday") {
  remove_filter("get_the_excerpt", "mfp_Add_Text_To_Excerpt");
}

// Take the excerpt, add some text before it, and return the new excerpt
function mfp_Add_Text_To_Excerpt($old_Excerpt)
{
  $new_Excerpt = "<b>Excerpt: </b>" . $old_Excerpt;
  return $new_Excerpt;
}