Calendar feeds
Put your fixtures in the calendar you already use (Google Calendar, Apple Calendar or Outlook), and they stay up to date by themselves.
Your own fixtures
Open Calendar in the sidebar and press Copy Link. In your calendar app, choose "Subscribe to calendar" or "Add calendar from URL" and paste it. Every fixture you're in, across all your leagues and cups, appears there, along with your competitions' meetings. When a fixture moves or a new one is added, your calendar catches up on its own.
The link is private to you, so treat it like a password and only paste it into your own calendar.
A whole league
A league with a public page also has a calendar feed of all its fixtures and meetings, for a club noticeboard, a venue, or fans who follow the whole league. You'll find it on the league's public page.