Subscribe to a Calendar

Subscribe to events from an external calendar that supports the iCalendar (iCal) format. To use this feature, go to Settings —> Subscribe to a Calendar.

Finding your external calendar’s iCal link:

Google Calendar

See section: “Get your calendar (view only)”

Sync your calendar with computer programs

You can get your Google Calendar events with other computer applications, like Outlook. Important: As part of a recent security enhancement update, the Google Calendar settings page has been improved to ensure that the secret address is now displayed in the hidden format.

support.google.com

Microsoft Outlook / Office 365

Under the settings in Outlook on the web, go to Calendar → Shared calendars

Introduction to publishing Internet Calendars

Internet Calendars are calendars that you publish to an Internet site, where others can view it or subscribe to it. Internet Calendars use the iCalendar format and the .ics file name extension. You can share one-time static calendars, known as Calendar Snapshots, in an email message.

support.microsoft.com

iCloud Calendar (Mac)

See section: “Share a calendar publicly”:

Share a calendar on iCloud.com

Invitees to a private calendar must have an Apple ID and use iCloud. In Calendar on iCloud.com, click to the right of the calendar name in the sidebar, then select Private Calendar. Type the invitee's email address in the Add Person field, then press Return or Enter.

support.apple.com

iCloud Calendar (iOS)

See section: “Share a read-only calendar with anyone”

Share iCloud calendars on iPhone

Tap Calendars, then tap next to the iCloud calendar you want to share. Turn on Public Calendar, then tap Share Link to copy or send the URL for your calendar. Choose a method for sending the URL-Messages, Mail, and so on.

support.apple.com

Basecamp

Schedule

Use Basecamp 3's schedule to keep track of dated to-dos and upcoming events with your team. Click on the Schedule tool of a particular team or project to get started (it isn't possible to add events directly to Upcoming Dates or My Schedule).

3.basecamp-help.com

On the “Pick a Calendar” page, you can get the ICS link from either Google Calendar or Apple Calendar.