Quatrain 39

Yet Ah, that Spring should vanish with the Rose!
That Youth's sweet-scented manuscript should close!
The Nightingale that in the branches sang,
Ah whence, and whither flown again, who knows!

FitzGerald, Stanza XCVI, 4th ed.

Both Heron-Allen (141) and Arberry (236) identify the Calcutta MS as the source, Heron-Allen C 223 and Arberry C 328: 

افسوس که نامه‌ٔ جوانی طی شد 
وین تازه‌بهار ارغوانی دی شد
آن مرغ طرب که نام او بود شباب
افسوس ندانم که کی آمد کی شد

afsus ke naame-ye javaani tey shod
vin taazebahaar-e arghavaani dey shod
aan morgh-e tarab ke naame u bud shabaab
afsus nadaanam ke key aamad key shod

Alas, the scroll of Youth rolled up so fast
Those vernal days of life not long did last;
The Bird of Gaiety whose name is Youth

I know not when it came of when it passed.

Saidi, quatrain 140

Arberry, Romance ... 236, refers to an emendation in the Calcutta MS made by FitzGerald: "FitzGerald correctly conjectured that shitaab [shetaab - "speed/swiftness"] was a slip for shabaab [youth]."  FitzGerald in a letter to Cowell (June 29, 1857, Terhune 2. 282) mentions this quatrain (his copy shows the quatrain number as 227) and says that he "can't make out what the name of the Bird of Joy in line 3 is".  I can't (yet) find evidence that he consulted Cowell further about this line.  Perhaps he made this choice of shabaab on his own.  FitzGerald, well-tutored by Cowell, was a pretty good linguist and was surely confident enough to venture a reasonable conjecture.

Although the Calcutta MS reading above has "roseate spring" (bahaar-e arghavaani), Forughi (63) and Hedaayat (35) have bahaar-e zendegaani "life-giving spring" which did not long last with life now in its decembrish mood. The winter solstice, the shortest day, usually falls on the first day of the Persian month دی/dey.

taazebahaar in the second line of the quatrain is a compound.  It means  something like "having the fresh bloom of spring" and falls into the bahuvrihi category of compounds.

Viewers will note the mellowing of FitzGerald from the last two stanzas.  There is resignation to fate towards the end of his poem