Day of joom-a-tee

Farabi

Here is as far as I understand about verse 629.
After Im asking to syaikh google to give me a hint (I tried to ask to a statue but I give up laugh ), finally I got some clue to solve this mistery.
Jama'a meaning is congregate, as you already know. The arabic language, is a gender distinction. To determined a words gender, we can see it from the character tha ( ) behind the word. And for the plural meaning we can determined it from the diacritic ( - I forget what the name is ) .

In here, we can see how the words jamaa'a mean plural.

738
Qa A La
A D Khu Lu W A
Fi Y
u Ma Min
Qa D
Kha La T
Mi N
Qa B Li Ku M
Mi N
A L Ji Ni
Wa A L i N Si
Fi Y
A L Na A Ri
Ku La Ma A
Da Kha La T
u Ma Tun
La 'a Na T
u Kh Ta Ha A
Ha Ta
i Za A
A Da A Ra Ku W A
Fi Y Ha A
Ja Mi Y 'a an
Qa A La T
u Kh Ra A Hu M
L u W L a Hu M
Ra Ba Na A
Ha L A A
a Da Lu W Na A
Fa Aa Ti Hi M
'a Za A B an
Di 'a F an
Mi Na
A L Na A Ri
Qa A La
Li Ku Lin
Di 'a Fun
Wa La Ki N
L a
Ta 'a La Mu W Na

So do in here

Wa A 'a Ta Si Mu W A
Bi Ha B Li
A L L Hi
Ja Mi Y 'a an
Wa L a
Ta Fa Ra Qu W A
Wa A Z Ku Ru W A
Ni 'a Ma Ta
A L L Hi
'a La Y Ku M
i Z
Ku N Tu M
a 'a Da A A
Fa a La Fa
Ba Y Na
Qu Lu W Bi Ku M
Fa a S Ba H Tu M
Bi Ni 'a Ma Ti Hi
i Kh Wa A N an
Wa Ku N Tu M
'a La
Sya Fa A
Hu F Ra Tin
Mi Na
A L Na A Ri
Fa a N Qa Za Ku M
Mi N Ha A
Ka Za Li Ka
Yu Ba Yi Nu
A L L Hu
La Ku M
Aa Ya A Ti Hi
La 'a La Ku M

And even a pair of human can also called ja-mee-'an. It can be seen in here.

238
Qu L Na A
A H Bi Tu W A
Mi N Ha A
Ja Mi Y 'a an
Fa i Ma A
Ya A Ti Ya Na Ku M
Mi Ni Y
Hu Dan
Fa Ma N
Ta Bi 'a
Hu Da A Ya
Fa L a
Kha W Fun
'a La Y Hi M
Wa L a
Hu M
Ya H Za Nu W Na

So what Im going to say is, the "double fattah" diacritic after we seen the pattern from the verses above had a meaning of plural.

What about "The day of assembly"?

Ya W Mi
A L Ju Mu 'a Ti

Day of assembly or Yawmi aljumu'ati is a single feminim form, so my hypotesis is, it point into day name.
Thats is it.

Well, as you can see, Im not as smart as I think, so correction is welcome.

Farabi

Ups, Im making mistake.
Jami'an didnot become plural because of the "double fattah" diacritic, but because it was a plural because of the form.

kata yang memiliki makna sangat kata ini memiliki makna sangat atau maha dicirikan dengan adanya huruf kasrah panjang atau huruf dhomah panjang yang ada pada konsonan ke 2 (k2). kata ini jika diawali dengan kata sandang al ( ) sering (dalam artian tidak selalu) digunakan untuk menerangkan sifat tuhan pencipta, pemelihara dan raja seluruh alam semesta. adapun jika tidak diawali dengan kata sandang al ( ) maka hanya digunakan untuk menerangkan sifat dari yang ada di alam saja.