A new and more secure main URL for the Google Calendar is set to be rolled out soon.
Over the next few weeks, the url will transition from https://www.google.com/calendar/ to the simpler calendar.google.com.
The overall impact of this change on Google Apps customers should be minimal since the old Calendar url will simply redirect to the new one,
However, there are may be few complications, Chrome extensions that depend on the old Calendar URL might not work properly if developers don’t make the necessary updates, and people who elect to get “gentle notifications” from Google Calendar will have to give the app permission in order for it to work again under the new URL.
The mobile apps won’t be affected by the URL change.