Because it's spelt "do" in Russian. It's only due to stress and the fact it's a preposition that the way it is said becomes "da".

As the preposition is so small, it's considered together with the following word, which in the genitive has its stress on the "а".

English. The answer is almost always English, and its "quirky" way of transliteration.

I don't think so. From what little Russian I know, the original is до свидания, with an O, but in most dialects an unstressed O is pronounced A: https://en.wikipedia.org/wiki/Russian_phonology#Unstressed_v...

It's even more convoluted than that. Due to vowel reduction, unstressed o is pronounced as a, but до is one syllable so the o is stressed. When pronouncing до separately, it would definitely be pronounced as do, but до свидания is always said quickly as if it was one word, so the до turns into an unstressed syllable.

As a side note, Russian pronunciation is remarkably uniform compared to languages like English. Dialects do exist but in practice the vast majority of speakers will pronounce things very similarly. The phonetics are also so different from English than the typical English speaker will not pronounce any Russian words close to native pronunciation.

> As a side note, Russian pronunciation is remarkably uniform compared to languages like English.

AFAICT, that goes for absolutely every other language too. Nothing even comes close to English for inconsistency in pronounciation (and even more so, spelling).

Interesting, thank you. Years ago I tried to learn Russian by myself, but it was quite difficult and I was too busy with other stuff. I hope one day I can learn it.