If you want to translate all the way into English, you could say something like "Festival of Vega" (which is the Greek name for the star celebrated), or you could just translate it directly from the Kanji and say "Night of Seven." If you did that, you would probably want to explain more about the celebration with a few sentences. If I were trying to mention it without explaining, though, I would say "Tanabata day," to English speakers.

Chinese Valentine's day

qixi festival ..?

Tanabata festival

Why not just call it Qixi since there is no equivalent found in English (functionally it is Valentine's Day but it still has its own origin and cultural expressions). If you still want the Chinese flavor of it, use the pinyin that represents the Chinese characters, after all, Tanabata is like pinyin for the Japanese characters, not very English but only transcription of the word into the Latin alphabets.