This question already has an answer here:
If a Jew lives outside of Israel, they should start saying "וְתֵן טַל וּמָטָר לִבְרָכָה" ("And give dew and rain for a blessing") during evening prayers on a certain evening in December.
Various sources disagree on which evening this should be.
The two competing opinions are as follows:
From an Ohr Somayach website article, it looks like we should usually start saying the words during the evening of December 5, just like The Book of Our Heritage says to do.
But I also looked at a blog article by Michael Laitner, which quotes Philip Baigel and Russell Grossman. That article seems to imply that we should usually start saying the words during the evening of December 4. I looked inside The Expanded ArtScroll Siddur (Sep. '10) and it agrees.
I haven't yet checked the online Metsudah siddur.
In practice, on which evening should we start adding the words "Vtein tal umatar livrachah" to our prayers?
Edit: Why I think this question should be reopened
I see that msh210 believes that this question is a duplicate of "How to calculate 60 days from tekufat tishri for any given year". Unfortunately, I've looked at yydl's answer there but am still confused.
That thread is good for more-technical readers. Still, I think we should leave this thread open for simpler folk like me, who want a simple, easy-to-understand answer. We can mark this thread as related to that one, instead of marking this thread as an exact duplicate of that one. (I've done so already; please see the "Related" section at the bottom of this post.)
Second edit: More reopen reasons
I don't need a fancy algorithm which will work for centuries; I'd just like simple instructions which will work for at least a decade or two.
This question isn't an exact duplicate of any other question; it's only similar. And the "Dr. Strangedupe" blog post by Jeff Atwood makes clear that it's okay to have a small number of open duplicates of a question, as long as the duplicates are similar but not identical.
Please see also the related thread: "How to calculate 60 days from tekufat tishri for any given year".