Connected on List Picker
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Click on the Edit button;

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Mouse hover the Web Part and click on the [+] button to add a new Event
- Fill out the form that pops up. You can check what you need to do in each section on the Calendar;
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After setting everything up, click on Save.

Directly on the Calendar list
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Open the settings menu and click on Site Contents;

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Look for your Calendar list and open it;

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Then, search for the day where you want to add an event, and click on Add.

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Fill out the form that pops up. You can check out what you need to do in each setting in the Calendar section of this User Guide;
- After setting everything up, click on Save.