31:
What happens once during a century, (Some version of 31 years... 1931 for example)
Never during a year, (There are never 31 months during a year)
Often during a month, (There are more often months with 31 days)
Never during a week, (A week never has 31 days)
Never during a day, (A day never has 31 hours)
Once during an hour, (An hour always has 31 minutes)
Once during a minute? (A minute always has 31 seconds)
