Q: it is about night
A: each night we count a certain number. These events are modern day events: Yom Hashoah (holocaust day), on the 13th count commemerates Holocaust Day for the Jews. Yom Hazikaron (day of rememberance) on the 20th count commemorates the wars fought for Israel. Yom Haatzmaut on the 21st count, celebrates the day Israel became on official state. Lag B'omer (33 of omer) is the 33rd count and celebrates that the students of Rabbi Akiva stopped dying.