[Vuejs]-Changing state from component to component

The solution to this depends on how distant your components are. Do they share the same parent? Then they can communicate through event emitting and props:

parent component
    calendar > $emit('setDate', date) // emits event with the date to parent 
    sidebar  > :selected-date="date"  // receives date via prop from the parent

If the components share no parent or are really far from each other (the common parent is 2 or more components away) some kind of state management like Vuex (https://vuex.vuejs.org/guide/) makes life easier.

This article explains it quite well: https://www.smashingmagazine.com/2020/01/data-components-vue-js/