Business side of your organization sets the calendars of all others. This can happen directly or indirectly, but all functional areas need to be aware of the core mission of their business and act in support of it.
In my particular case, in higher education, academic calendar sets our IT calendar and has a significant impact as to how and when we proceed with some of our responsibilities. This particularly comes into play during midterms and finals as well as during important events across campus. During these times we can’t have any planned outages (unplanned outages are a different beast entirely) as they would negatively impact the primary mission of our university, student education. Same goes across most industries and as such it is essential to be aware of the primary calendar of events in order to continue to provide value to the entire organization.
Communicate, collaborate and work together towards common vision and goals.