Okay so this is an old thread but I'm going to respond anyway because it's interesting and nobody else said this.

"Calendar day" means whole days - midnight to midnight.

So for example if a website says "Once you begin your application it must be submitted within 3 calendar days" this means regardless of when on the first day you start it, you have until midnight on the third day to finish.

This is different from 'days' which could ambiguously mean either whole days or periods of 24 hours, depending on interpretation.

So "calendar day" is a more specific term intended to resolve that ambiguity.

(And yes, it does includes weekends and other non-working days)