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31st day of the month
31 days
三十一日が大晦日にあたる年末は忙しい。
The year-end when the 31st falls on New Year's Eve is busy.
今月は三十一日まであるから余裕がある。
This month goes to the 31st, so there's some leeway.
三十一日に届く予定だった荷物が遅れている。
The package that was supposed to arrive on the 31st is delayed.