[Avodah] Not saying Tachanun (was Yom Haatzmaut)

[Ben Waxman via Avodah]

Putting the question of Hallel aside, I don't understand the line 
below.  I looked through the Beit Yosef OH 131 where he discusses the 
days on which Tachanun isn't said. The BY does not list all the days 
that we skip (like Purim Qatan or Pesach Sheni). These came later.

So when did this idea (that we can't change when Tachanun is said or not 
said) start?

Ben

On 4/28/2015 12:31 PM, Prof. Levine via Areivim wrote:
 > Shemone Esrei is followed
 > by Chazaras Hashatz, which is followed by Tachanun, and then followed
 > by Ashrei and U'va' Letzion--and we do not have the right or
 > privilege of changing that, he opined.