Why doesn’t my iPad calendar synch with my iPhone?

Is your iPhone also set to synch the calendar? If not set it up. If yes, please look at this discussion: https://discussions.apple.com/thread/8474050?language=en

Did you set iCloud on your iPad? If not go to Settings, tap your name and tap iCloud. Then turn on the data you want to be synched, e.g. Calendar in your case.