The first column on a monthly calendar is a habit, not a law. Many U.S. wall calendars put Sunday first. Many office planners and most ISO week tools put Monday first. This site offers both because a printable calendar should match the household or the workplace that will write on it.
Sunday start spreads the weekend to the two ends of the row: Sunday on the left, Saturday on the right. Monday start groups Saturday and Sunday on the right. If you plan weekends as a block, Monday start is easier to read. If you grew up reading Sunday-first kitchen calendars, Sunday start will feel native.
January 1 does not move into column one unless January 1 falls on that weekday. The start setting chooses the weekday headers. The dates follow the real calendar.
ISO week numbers always use a Monday-based week. You can still print those numbers on a Sunday-start month. The gutter then labels ISO weeks while the columns stay Sunday-first. That mix is honest, and it can look odd if you expected the gutter to count Sunday-start weeks. If the mismatch bothers you, use Monday start when ISO numbers are on.
Weekly layouts follow the same control. An hourly week that starts on Monday is a work timetable. An hourly week that starts on Sunday is a household week that includes the weekend at the left.
Tuesday through Saturday are in the same control. A Friday-to-Thursday shift, a Saturday-first household, or a Wednesday payday week can put that weekday in column one. The dates do not move. Only the headers rotate. The week-start page opens on a Saturday-start month so you can see a first column that is not Sunday or Monday.
There is no moral ranking. Print a sample month both ways if the household disagrees. Paper is cheap compared with a year of talking past each other at the fridge.