The calendar lists the delivery dates of the month that fall on the fulfilment weekdays configured for the selected kitchen location, for the menu type you are on:

• Meal Prep Menu and POS Menu use the kitchen's Meal Prep fulfilment weekdays.
• Catering Menu uses Catering fulfilment weekdays.
• Real Time Menu uses Realtime fulfilment weekdays.
• In Store Menu uses In Store Menu fulfilment weekdays.
• Wholesale menus use Retail / Wholesale fulfilment weekdays.

These weekday checkboxes live on the kitchen location edit page (Kitchen Locations). Dates on other weekdays are not offered on the menu page, and cloning also skips them. The underlying delivery dates themselves must exist too - see the Kitchen Locations & Delivery Dates article.