To enable calendar view for your release and version click on “Calendar” in top navigation bar
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All versions and releases from the board that have Release Date specified will be shown on the Calendar view.
versions are shown Green
release are Blue (thus we are consistent with other view through out the App)
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Please follow the following instructions from Atlassian to create your confluence calendar
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https://confluence.atlassian.com/teamcal/create-add-and-edit-calendars-654213559.html
Click Subscribe by URL, enter URL you copied and authorization details
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to get one
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Calendar as Gadget?
Sure, check respective section of this documentation.