Since the book takes place in late August through early September that means that the curtis parents most likely died on or around Darry's 20th birthday.
The exact date is never mentioned but we can get a rough estimate of the time based on the fact that Pony said he had been 14 for a month, his birthday being on the 22nd of July. Soda said later that he would be 17 in a few weeks and his birthday being October 8th.
Now the curtis parents died eight months before the book take place. August being the 8th month of the year place their death in and around January. Darry's birthday is January 5th.
