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Be careful with this. Importing transits into your default calendar can create an overloaded calendar. 

1) Login to Gmail or Google. 

2) Create a fresh, empty, new calendar in your Calendar app.
Then import the calendar ical/ics formatted file into that new empty calendar.
If you skip this step, the calendar will import into your default calendar.
To undo that - you will have to delete the default calendar.

By creating a new, separate calendar - you control when and where you see it, and delete it easily if needed.

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