This is what the Lexicon says on Lily's page:
Lily and as a matter of fact, Alice too, is likely to have been pregnant around Oct-Nov 1979, right? So, I'm confused here, utterly!
The prophecy foretold the birth of a boy "with the power to vanquish the Dark Lord" at the end of July, so as soon as Lily became pregnant the Potters went into hiding (OBT/CH). Voldemort waited to see what children were born, and began planning the murder of the Potter family (OP37, HBP25).
Prophecy came the following year, either very late 1979 or early 1980, hmm?
Voldemort waited to see what children were born... and so if Lily and James were hiding, wouldn't it be likely that Dumbledore ask the Longbottoms to be careful enough too? Frank and Alice were in the Order. If Voldemort waited, there is no way for Dumbledore to be sure that its the Potter's child Voldemort is after.
Maybe I'm focusing too much on Lexicon which despite being reliable can be a bit confusing, leading to misinterpretation.
I've the following events in my mind:
- The prophecy,
- Lily becoming pregnant
- when Potters went into hiding,
- when Dumbledore told the Potters about the prophecy,
- when Voldemort decided it was the Potter's child he wanted,
- when Snape came to Dumbledore with information of Voldemort's intention to go after Potter's.
Giving exact date is difficult obviously. I understand also that canon gives no exact evidence for any of this. But we can debate, right? Could you arrange the above in the order it is most likely to have occured.
Here are some relevant excerpts I found:
"The odd thing, Harry, is that it may not have meant you at all. Sybill's prophecy could have applied to two wizard boys, born at the end of July that year, both of whom had parents in the Order of the Phoenix, both sets of parents having narrowly escaped Voldemort three times. One, of course, was you. The other was Neville Longbottom."
OotP, page 742 - British version
"He chose the boy he thought most likely to be danger to him. And notice this, Harry: he chose, not the pureblood (which, according to his creed, is the only kind of wizard worth being or knowing) but the half-blood, like himself. He saw himself in you before he had ever seen you, and in marking you with that scar, he did not kill you, as he intended, but gave you the powers, and a future, which have fitted you to escape him not once, but four times so far - something that neither your parents, nor Neville's parents, ever achieved."
OotP, page 743, British version
"He heard only the beginning, the part fortelling the birth of a boy in July to parents who had thrice defied Voldemort. Consequently, he could not warn his master that to attack you would be to risk transferring power to you, and marking you as his equal. So Voldemort never knew that there might be danger in attacking you, that it might be wise to wait, to learn more. He did not know that you would have the power the Dark Lord knows not -"
HBP, page 512, British version
"Professor Snape made a terrible mistake. He was still in Lord Voldemort's employ on the night he heard the first half of Professor Trelawney's prophecy. Naturally, he hastened to tell his master what he had heard, for it concerned his master most deeply. But he did not know - he had no possible way of knowing - which boy Voldemort would hunt from then onwards, or that the parents he would destroy in his murderous quest were people that Professor Snape knew, that they were your mother and father - "
DH, page 676, American version
"The corridor dissolved, and the scene took a little longer to reform: Harry seemed to fly through shifting shapes and colors until his surroudings solidifed again and he stood on a hilltop, forlorn and cold in the darkness, the wind whistling through the branches of a few leafless trees."
-Akay - who thinks she might've gotten her hands on a Befuddlement Draught!