What surfaces you can & can't have your MacBook on.

Desk calendars are fine. Any hard surface is fine. Soft surfaces such as beds which block airflow are not good surfaces to place your computer on.

From: About fans and fan noise in your Mac - Apple Support

"The surface your Mac rests on can block its vents, affecting fan behavior. If you use your computer on a soft surface like a couch, pillow, bed, or your lap, its fans might run more. Using your Mac on a hard, flat surface like a table or desk is best for temperature control."

Calendar and other pads are not the best surfaces. Solid material like a plain desktop or table are preferrable. If you must use it on your desk calendar (or really on anything) is laptop pad that will elevate the rear of the computer enough to permit ventilation.

This is an inexpensive way to go. It's also portable and doesn't require much space. Water & Wood Notebook Laptop Heat Reduction Pad Cooling Cool Feet 4 Pcs. You only need use two of them.

In a pinch I've even used a pair of Coke plastic bottle caps in the rear corners to lift it up and enhance airflow underneath.

But yeah, formal laptop cooling pads are less unsightly. And some fancy ones can even have auxiliary fans that help quite a bit when running really intensive apps such as video renders, fancy Photoshop filters and the like.

When I want to Mac-in-bed, have a rigid board on top of which I place a RoadTools CoolPad and then the Mac.

Get yours before they run out: https://eshop.macsales.com/item/Road%20Tools/RTCP/