More context is needed. (real situations) -1- At the meeting tomorrow, I will say X. -2- If I go to the meeting tomorrow, I will say X. (unreal situations) -3- If I went to the meeting tomorrow, I would say X. -4- If I had gone to the meeting yesterday, I would have said X. variations of -3- If I were/was you, I would say X. If I were/was in that situation, I would say X. I would say X. [implied - If I were/was ...] More information about real and unreal conditionals. https://www.ef.com/ca/english-resources/english-grammar/conditional/ Reported speech. Yesterday: "John, I will say X at the meeting." Today: "Yesterday, I told John that I would say X at the meeting." More information about backshifting verbs. https://www.ef.com/ca/english-resources/english-grammar/tense-changes-when-using-reported-speech/

Depending on the context that phrase could be correct. It can be used when correcting someone ex: I would say ____ instead of ____. In this context it’s a polite way to suggest an alternative. In What context are you looking to use the phrase?

I’m very thankful to you all for your answers and explanation.

I don't see anything wrong with it though. Especially if it's used in a conditional sentence. example: "If you met him, what would you say to him?" "I would say to him that I don't like his attitude."

It depends on the contexts of what you are trying it say. There are instances where it is correct and instances where it is not correct.