It is based on current date and time. {{#time: Ym|+1 month}} changes to 201712 and page Calendar 201712 is added to the page.
It changes automatically.
and if you want to see the 3rd month out : then use =
{{:Calendar {{#time: Ym|+2 month}}|Week}} - - cheers - ja
the Main calendar is at = https://www.wikitree.com/wiki/Help:Calendar -
Top RH corner = Drop down - [ Calendar ]