Visit Google Calendar and, under My Calendars, click the three dots (⋮) next to the calendar you want to share.

Select Settings and sharing.

Note: To embed a Google Calendar on your site, it must be set to public. If the calendar is private, it will not be viewable by others.

To make Your Calendar Public: Go to Access permissions for events, check the box that says Make available to public, and confirm your selection.

Scroll down to the Integrate calendar section, then copy the Public URL to this calendar.

Paste the copied URL into the Google Calendar URL input field in the Settings panel, and click Connect.

To add more calendars, simply paste the URL in the same field and click on add calendar. It will be shown in the calendar list.