Commerce: Calendar to Product Quantity
Sounds like a fun project. :) My thoughts:

– I'd ditch using Commerce' stock management, and set all products to have unlimited stock.

– I'd build a plugin that has a field type that attaches to products/variants, that lets the client add stock to certain dates for that product/variant.

– I'd have template variable for getting stock for a given date and product/variant, and use it to show availability in the templates. If you need to show availability in product listings, I'd probably also create a custom template variable for getting products (instead of using craft.commerce.products directly), that extends the element criteria to add your custom stock availability to it (see this article, or my Similar plugin for examples).

– I'd listen for some Commerce events (commerce_orders.beforeOrderComplete and commerce_variants.onOrderVariant) to update the stock level when an order is placed (see the Stock Notifier plugin from P&amp;T for examples)

To be honest, I think the biggest challenge is the second point, making the interface and database structure that lets the client update availability. A real simple solution where they add a date and a number for stock, is easy enough. But making a nicer calendar interface, and adding support for stuff like recurring dates ("we have 20 fresh cinnamon rolls every thursday"), gets pretty complicated pretty fast. But, it all depends on the use case how advanced it needs to be.

Anyways, that's my initial thoughts. :)