Hi,
The check is not to apply the discount only on the products of "1-Day", that's already the case, it's to check the amount of the products of the "1-Day" category in the cart to know if we apply or not the coupon 
A little plugin should be sufficient, the plugin must be based on the function "onBeforeCouponLoad(&$coupon,&$do)" and you have to get the cart content thanks to
$cartClass = hikashop_get('class.cart');
$fullCart = $cartClass->loadFullCart();
Then check the products and set "$do" to false if the conditions are not met, or true to apply he coupon.
Here is some documentation:
www.hikashop.com/support/documentation/6...entation.html#coupon