Translation:I have been looking forward to this day for fifty-two years!
Both "už" and "se" want to be in the 2nd position, so they "fight" for it. They happen to have a similar priority (or "fight power", if you will), that's why it doesn't matter which one wins. The one that "loses" will go next.
- Na tenhle den se už těším X let.
- Na tenhle den už se těším X let.
Both word orders are correct and natural.
"Už" is actually quite flexible, we can also place it before the numeral if we want to emphasize it: "Na tenhle den se těším už padesát let". Or we can start the sentence with it, "už" really doesn't mind: "Už se na tenhle den těším padesát let." or "Už padesát let se těším na tenhle den." or "Už padesát let se na tenhle den těším." - in the last two examples, "už padesát let" forms a tight unit that takes up the 1st position.
We can also start with the verb "Těším se..." for several more possible word orders. Not to mention starting with "Já...". So, as we can see, there are lots of options!
The situation would be different in the past tense, for example - because the auxilliary verb (form of "být") has a high priority - it has more muscle and will always go to the second position (or we can say, the start of the 2nd position). The reflexive "se" will jump on its back and follow immediately, leaving the weaker "už" well away from the "sunny spot":
- Na tenhle den jsem se už těšil padesát let.
- Na tenhle den jsem se těšil už padesát let.
- ...plus varieties with "už" as the first word or "Těšil jsem se..."
You're welcome. One more thought: when I called "už" weaker, I only meant "weaker" in terms of its fight power to get into the 2nd position. In fact, words away from the 2nd position are generally stronger in their meaning, while the little words in the 2nd position are mostly helper words. Therefore, when "už" is NOT among the 2nd position words, but instead at the beginning of the sentence of right before the numeral/amount, it's a bit more emphasized. Compare: "Já ho vidím." (I see him.) and "Já vidím jeho." (It is him that I see., here the 2nd position is empty)
Also, the word order to follow when in doubt is SVO (subject verb object). VSO is a typical question word order. It's not very relevant here though, since the subject (já = S) is not expressed and any word order where "na tenhle den" ( = O) comes later in the sentence than "těším se" ( = V) is automatically SVO. The main translation is not SVO, it fronts the object by making it the topic of the sentence.
It's two different philosophies of looking at things. English grammar says: "The action is still going on BUT we need to emphasise that it's started in the past" while Czech grammer says "The action has started in the past BUT we need to emphasise that it's still going on". So the English construction of "since" + present perfect very likely takes present tense in Czech.