There is no information about Allbirth
Even if you look into the Inner Sea Gods, it has no description or clarification of what that holiday is about, all we got (p.97) is the following on Lamashu's Holidays:
The cults of Lamashtu celebrate no known regular holidays, though they engage in debauched hedonism to celebrate births, deformities, and demonic visitations. The entire month of Lamashan is sacred to them, not only because it is named for their goddess, but because it represents the transition from fall to winter, a time of year when the weakest offspring die from the first seasonal illnesses.
With a similar text, this is what we got a few years earlier in Faiths of Corruption, on Religious Holidays (p.30):
The month of Lamashan is named after the Mother of Monsters, for it is said to be a dark month when unholy creatures spawned in the deadly cold of winter and in the fevers of spring tear themselves from the womb. The faithful of Lamashtu do not celebrate the month in any way greatly different from any other month. Instead, they mark occasions as the mood takes them, gloating over their dying enemies or holding up the latest deformed child as evidence of their goddess’s favor. They live by no set schedule, and instead offer their bloodied hands as often as they deem fit.
The only mentions of Allbirth in the books are from the 3.5's Gods & Magic about Golarion's holidays, which was later replicated to the Inner Sea World Guide almost as is. The fact that this holiday is listed is in direct contradiction with the text of Inner Sea Gods though.
That said, there is a high possibility that Allbirth was discarded because of lack of interest , as I couldn't even find many topics about it on Paizo's messageboards. But, if you are interested, I found this article about it on a fan-made (as no source is given) website.