プラグインが有効かどうかで分岐

 <?php //営業日カレンダー ?>
 
<?php include_once( ABSPATH . 'wp-admin/includes/plugin.php' ); ?>
<?php if(is_plugin_active( 'business-calendar/business-calendar.php')): ?>

<div class="top_cal "> 
<div style="text-align:center; font-size:12px;">営業日カレンダー</div>
  <?php echo do_shortcode( '[business-calendar num=1 past=true future=true event=true]' ); ?>
</div>
<?php endif; ?>

https://www.imamura.biz/blog/function_reference/is_plugin_active

よく使うなら
ヘッダに

<?php include_once( ABSPATH . 'wp-admin/includes/plugin.php' ); ?>

かも