Part of the weekday Shachrit includes Psalm 100, which is מזמור לתודה - a song for soda.

We don't sing about soda on Shabbat, Yom Tov, Erev Pesach, Hol Hamo'ed Pesach and Erev Yom Kippur.

Many people like soda. Some people sing whenever they eat or drink, and bubbly water is refreshing and gladdens the spirit and the esophagus. It's something to sing about. And, I know we're allowed to drink soda on all these days except Yom Kippur. (OK, I can understand, then, why it's eliminated then.)

So what's special about all the other days when we can't sing about soda?

This question is Purim Torah and is not intended to be taken completely seriously. See the Purim Torah policy.

It's obvious. You should be drinking wine on those days - not soda. As for Erev Yom Kippur, you should be drinking wine then too in order to make the transition from drinking to fasting harder and therefore intensifying the "inui" affliction.

Well, we really have to start with the reason why we are always guzzling soda like, well, soda. And the reason is because of what you mentioned, that it gladdens the spirit, and helps us to be happy, as the psalm there continues (verse 2) עִבְד֣וּ אֶת־יְהֹוָ֣ה בְּשִׂמְחָ֑ה, serve Hashem with happiness. That is, we are singing about the soda because it enables us to serve Hashem with joy.

On days when we anyways have joy, there is no need for soda. Therefore, on Shabbos and Yom Tov with the festive meals¹, and erev Yom Kippur where we are already happy about our upcoming atonement, and on erev Pesach when we get to stuff ourselves with all of the junkfood we can fit into our mouths under the guise of "getting rid of it before it's too late," we don't have to resort to soda to bring us happiness.

¹ and on Chol HaMoed Pesach, when we have all of the leftovers that we didn't eat at the seder because we forgot that by the time Shulchan Orech rolls around at 1:38 a.m. no one will be in the mood to eat a whole meal.

It's because תודה reminds us of סודה, which reminds us of the founding of Mitzrayim, which is a sad time and not a happy time. If Mitzrayim had never been founded then an evil Paro would not have arisen to enslave us all, and we would not have been driven down so far that we fell into Mitzrayim's idolatrous ways mere weeks after receiving the torah. Mitzrayim brought us sorrow and woe, and we do not want to be sad on festive days! And even on Yom Kippur, we should be reflecting on our own deeds, not Mitzrayim's.

So why is it permitted to sing of soda on other days? On other days we hear of sadness and evil all the time (at least if we are within range of any news source). Singing that reminds us of Mitzrayim cannot possibly make things worse on those days; in fact, a little singing about Mountain Dew might distract us and singing of Red Bull might even elevate us. But don't sing of Sprites, though; that way lies trouble.