According to this lovely calendar provided by the Lexicon (
http://www.hp-lexicon.org/timelines/...lendar_dh.html) it would have been sometime in April.
Assuming that Lupin & Tonks got married sometime after Harry, Ron & Hermione left school (which I think is a reasonable assumption as Lupin was still pushing her away as recently as the night Dumbledore died), and assuming a wizarding pregnancy is the same length as a Muggle one, and assuming she carried the baby to term, she would have to have gotten pregnant pretty well close to the day they got married, because for a 280 day pregnancy, she would have to have been pregnant by mid-July to have the baby at the beginning of August, and they go to pick Harry up on the 21st of July, and they were already married by that point.
Further, Lupin knew about it by August the fourth.
Even assuming they got married on, say, June 20th and she got pregnant within three weeks, it's entirely possible that she wouldn't even *know* she was pregnant until a month later, giving her time to miss her menses. So the timeline is VERY tight.
And you can't make it much later than June 20th, because unless there's some sort of Wizarding knowledge that tells you that sort of thing within moments, she simply wouldn't have known.