My dad will often say "What's the order of the day?" It means what important things do we need to do today?

I believe this originates from military practice where an order is published, say on a Monday, that will notify all troops of the main activities that will be required on Tuesday and so on. This may include the standard uniform to be worn, where and when to report for duty and what type of work will be carried out. It has entered common usage but essentially indicates what, where and when things need to be done during the day.

can be something that is usual or expected, for ex. no context, no meaning.