PowerPoint doesn't feature a built-in option for inserting calendars into slides. You can, however, search for calendar templates online and then copy the slide containing the month that you want into your presentation.
Open a blank presentation in PowerPoint and click New under the File menu. Enter Calendars into the search box and press Enter. Select a calendar from the list of templates that display.
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Click Create on the pop-up window to download and insert the calendar template into the blank presentation.
Select the slide that contains the month you want from the left-pane, and then right-click and click Copy.
Calendar templates feature fillable fields under dates that you can complete before or after copying the slide to your presentation.
Head over to your presentation, and then right-click a vacant area in the left pane and select the Keep Source Formatting icon under Paste Options to insert the slide to your presentation. This option copies the calendar exactly as it is. Click and drag the slide up or down within the left pane to arrange it within the presentation.
Under Paste Options, click the Use Destination Theme icon to insert the slide and automatically convert it to adhere to the style of your presentation. Alternately, click the Picture icon to copy the slide as a picture into any other slide; use the sizing handles around the picture to adjust its size.