Megillah, Chapter Two, Mishnah 6. In the previous mishnah we dealt with mitzvos whose time is during the day, which are valid to perform throughout the entire day. Our mishnah deals with mitzvos whose time is at night.
The text of the Mishnah:
"Kol halaylah" - the entire night is valid for the following mitzvos:
"Liktzir ha'se'orah" - the harvesting of the grain, the barley that was used for the Omer offering on the sixteenth of Nissan. This harvesting is done at night, and it is valid all night long.
"U'lehaktir chalavim v'evarim" - the burning of the fats and limbs of the burnt offerings, which is likewise valid all night long.
"Zeh haklal":
From here the mishnah sums up a single rule that covers both categories:
"Davar shemitzvasah bayom" - something that the mitzvah is to do during the day is valid throughout the entire day.
"Davar shemitzvasah balaylah" - a mitzvah whose time is at night is valid throughout the entire night, and one is permitted to perform it at any hour of the night.
In summary: in this mishnah we learned the rule that completes the previous one - just as something whose mitzvah is by day is valid all day, so too something whose mitzvah is by night is valid all night, and this includes the harvesting of the barley for the Omer and the burning of the fats and limbs.