A question that comes up often is why Outlook's calendar doesn't mark all dates that contain appointments in bold in the small thumbnail calendars.

Outlook bolds the dates that contain appointments, for a period 10 months ahead and 2 months in the past (a total of 12 months). The reason for it is because it would take too long and require too many resources for Outlook to bold all dates containing appointments, especially on calendars that cover several years. Slower computers or calendars with a lot of current appointments already require several seconds to display bolded dates.

Can you configure it? No, it's hard coded and can't be changed.