last day of the month
「三十日」 has another reading with different meanings.
30th day of the month
end of the month
三十日には、特別な行事が毎年行われている。
A special event is held every year on the last day of the month.
家族は三十日に集まって、年末を共に祝う。
The family gathers on the last day of the month to celebrate the end of the year together.