k, turns out it was actually quite easy to find and yes, it is defenitly something to wonder about 🙂. here it goes:
Winning question & answer (with 60% of the vote):
What did Dumbledore's Howler to Aunt Petunia mean? ('Remember my last'?)
Well, it is a relief to move on after the Mark Evans fiasco. This time, two out of the three poll questions had interesting answers (or so I think) and thank goodness you chose one of them.
So: Dumbledore is referring to his last letter, which means, of course, the letter he left upon the Dursleys' doorstep when Harry was one year old. But why then (you may well ask) did he not just say 'remember my letter?' Why did he say my last letter? Why, obviously because there were letters before that…
Now let the speculation begin, and mind you type clearly, I'll be watching
P.S. It has been suggested that I am wrong in saying that Dumbledore's last letter was the one he left on the doorstep with baby Harry, and that he has sent a letter since then concerning Harry's illegal flight to school. However, both Dumbledore and I differentiate between letters sent to the Dursleys as a couple, and messages directed to Petunia ALONE. And that's my final word on the subject - though I doubt it will be yours 🙂