Meaning of にち in Japanese

この服は寒い冬の日には向かない。
These clothes are not appropriate for a cold winter day.

１３日の金曜日は不吉な日だと言われている。
It is said that Friday the 13th is an unlucky day.

その提案一石二鳥を狙ったものだろうけど、「虻蜂とらず」にならなければいいけどね。
That proposal may be a way to kill two birds with one stone, but we also have to be careful not to get greedy and spoil everything.

一日一日と寒くなってきた。
It is getting colder and colder day by day.

１日中テニスをしていたのだから。
He was playing tennis all day.