late in the month

I clean my room.

私は部屋を掃除します

I clean my room in the last part of the month.

私は下旬に部屋を掃除します

I clean my room in the last part of every month.

私は毎月下旬に部屋を掃除します

I clean my room with my younger brother in the last part of every month.

私は毎月下旬に弟と部屋を掃除します

I clean my room thoroughly with my younger brother in the last part of every month.

私は毎月下旬に弟と部屋をきれいに掃除します