Put the cursor in the where you want to insert a calendar in OneNote UWP.
Then, click Gem Menu -> "Insert" Menu -> Calendar -> "This Month" menu item.
Gem will insert this month calendar into OneNote UWP.
Gem Menu insert the calendar start from Sunday in default.
But, you can setting it start from Monday.
Click Gem Menu -> Insert -> Calendar -> check the "Week First is Monday".
Click "This Month" menu item again, it will insert a calendar start from Monday.