What is "Ad Avel ha'Gedolah"?
Rashi: This is "Even ha'Gedolah" above (verse 15); the Lamed is in place of the Nun. We find [conversely] that the Nun in "Nishkah" (Nechemyah 13:7) is in place of a Lamed. Because mourning resulted (verse 19), it is called Avel.
Radak: There were open cities of Pelishtim near where they put the Aron [in Beis Shemesh].
Malbim: It is the border of Eretz Yisrael.
What did they bring "me'Ir Mivtzar v'Ad Kofer ha'Perizi"?
Radak: It is as if it said "ume'Ir Mivtzar." The Kohanim commanded to bring only five Techorim, and mice, like the number of Sarnim and big cities. However, wherever the Pelishtim dwelled, also the small cities and open cities (without a wall) brought, for they were still afraid.
Malbim: The mice were on behalf of the people. Even though the Kohanim commanded to bring only five, every city brought. There was only one Techor for each noble.
Why does it say "Ad ha'Yom ha'Zeh"? The Aron did not stay there very long!
Radak: The Aron [exists] until this day [when Sefer Shmuel was written 1 ]. They put it on the great rock in Sedei Yehoshua.
Refer to Shoftim 1:21:4:1,2 and the note there.