How do you show which Sale a product has applied in Commerce? It's easy to know if a product has a valid Sale applied to it with product.defaultVariant.onSale - but how do you know which Sales are being applied to the product? How do you output that the product is currently part of a specific named sale, and any associated content you might want to show?
The real answer is:
{% for sale in variant.sales %}

    {% sale.description %}

{% endfor %}