Hi Creoacraer,
Thanks for posting in Apple Support Communities! You'll see these red lines if you've added certain calendars. Find more information below from the article Use a Chinese, Hebrew, or Islamic lunar calendar on Mac:
The Calendar app supports showing Chinese, Hebrew, and Islamic lunar calendars. When you turn on a lunar calendar, the lunar calendar dates are listed along with their Gregorian calendar equivalents. The first day of a lunar month is marked with a thin red line. The first day of a lunar year is marked with a thick red line.
If that's not the case, let us know if you see any events or holidays on these days. Which macOS version is installed under the Apple menu > About This Mac? What Calendars do you have selected? If you don't see them, select View > Calendar List from the menu bar.
Cheers!