When is a time to cry?
Rashi: Tish'ah b'Av.
Ibn Ezra: It is one who worries, to the point that he cries.
When is a time for laughter?
Rashi: "Az Yimalei Sechok Pinu" (Tehilim 126:2).
When is a time of eulogizing?
Rashi: Days of mourning.
Metzudas David: When one's relative dies, he desires to eulogize him, for this assuages the pain.
When is a time of dancing?
Rashi: It is for Chasanim and Kalos. Metzudas David, Metzudas Tziyon - or, it is when one wants to arouse Simchah. Rekod is jumping happily.
Ibn Ezra: It is with others.
Why does it say "Sefod" and "Rekod" without Lamed in front? All other 26 times are Makor (infinitive, beginning with Lamed)!
R. Avigdor: Both of these apply to "Es Livkos." When it is time to cry over a Mes and we eulogize him, the angel of death dances, for he has authority to strike. He dances in front of the eulogizers. This is why [according to one opinion in Yerushalmi Sanhedrin 2:4], women go first [in back of the bier], because they (Chavah) caused death [to come] to the world.
Kol Eliyahu: Lamed hints to Limud Torah. We are Mevatel learning to take a Mes out for burial, and for Hachnasas Kalah 1 .
Magihah: Why does it say "Es Likro'a (7)"? One who tears upon hearing that his relative died, he may not learn! In some cases, one may learn, e.g. he tears due to hearing blasphemy.