And the winner is...
moshe g. g.
and listed as his website is
who correctly answered that the day of the Hebrew calendar that some years we say Tahanun and some years we say Hallel (not having to do with Yom Ha'atzma'ut) is:
when some years Hanukah falls on that day and some years it does not, depending on whether there is 1 or 2 days of Rosh Hodesh Tevet. Again, the first commenter got it right. I probably need harder questions.
Now, I'm wondering, is this Moshe G. G. the same as Moshe Feiglin himself? If so, I feel extremely honored that he reads my blog. But even if it's a different Moshe, I feel honored that you read my blog, too. Please comment to let me know. Thanks.