Calendar Plant

I read the book The Serviceberry: Abundance and Reciprocity in the Natural World by Robin Wall Kimmerer.  In the very first pages, she talks about the serviceberry being a "calendar plant." 

Serviceberry is known as a calendar plant, so faithful is it to seasonal weather patterns.  its bloom is a sign that the ground has thawed. In this folklore, this was the time that mountain roads became passable for circuit preachers, who arrived to conduct church services. It is also a reliable indicator to fisherfolk that the shad are running upstream...."

Reading this made me wonder what it is that is a calendar for us. For me, there are planners, digital calendars, the rhythm of the ministry we do at the Foundation.  I sometimes bend my schedule to the weather - when it is snowy or rainy - but not often. What is it that tells us the pattern of our days?

With the abundance of grocery stores, we have vegetables and fruits all year round. I don't have to wait until they grow in my yard or for the farmer's market to bring them to sell to me. With cars and airplanes, my travel is rarely (although sometimes) dictated by the rain making mud or slick roads. Christmas comes in July and again in September, and if I don't hurry, I'll miss winter coats in the stores. We dictate our time, very often.

What would it be like to live in the rhythm of nature? To plan our days by a calendar plant?