how can I restore calendar app on my apple watch

Hi

When the Calendar app is removed from your iPhone, then it is also removed from your paired Apple Watch.

Follow the instructions here to add the Calendar app back to both devices (adding it back to your iPhone automatically also adds it back to your Apple Watch):

Delete built-in Apple apps on your iOS 11 device or Apple Watch - Apple Support

Go to the Apple Watch App on your iPhone....click on DOCK....click on Edit.....scroll down to Calendar....click on the Green circle on the left. It should now be in your Dock (unless you have 10 Apps in there already....if so....you will have to remove one).