So i have checked the settings for the native calendar app and there is nothing available to show the year or week numbers. It would be useful to show these at the side or top.
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There is no setting.
But you just have to tap the calendar anywhere and move it to the side (right for current, left for next) and you will see month, year and calendar week.
Not really intuitive but if you know 
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Never seen this! You make my day haha really.