>I want to know why the same kanji with the same meaning has two pronunciations. or you can think like this if you like. "Two different words share the same kanji for themselves" さくや and ゆうべ are almost the same but in details different. They are the same in the meaning of "last night", different in the range of time. 昨夜 is for "night", ゆうべ（夕べ） is mainly for "evening". >I want to know why the same kanji >with the same meaning has two pronunciations. This often happens in Japanese. Kanji for eyes and sound for ear, or kanji for the mind and sound for the heart. Why? I don't know. Our ancestors probably thought it convenient, fun, or beautiful.

You can pronounce "昨夜" as ”ゆうべ” and ”さくや”. Both are correct. ゆうべ can be written "夕べ" too. The kanji "夕” spelt "ゆう" in hiragana. "べ” is hiragana. 夕方（ゆうがた) is the time period from around 4 pm to 6 pm. So it's shorter than evening. The kanji "昨” is spelt "さく” in hiragana. 昨日 is can be pronounced both as ”きのう” and "さくじつ". "さくじつ" sounds formal whereas "きのう” is general. 昨晩（さくばん) is another word for 昨夜. The difference between "さくや" and "ゆうべ” would be ”さくや” is more formal whereas "ゆうべ” is more general or poetic. For example, if a news anchor were to report a traffic accident that happened last night, they would use ”さくや” instead of "ゆうべ”. For example, there is a song called "ゆうべのうた”. It's spelt 「夕べの歌」with kanji. "さくやのうた” is awkward for a name of a song.

I'd say that you more often read "昨夜" as "さくや" than "ゆうべ". ／"ゆうべ" has two meanings. One of them is "last night", which is almost the same as "さくや". In this case, you could write it as "昨夜", though many people probably read it as "さくや", not "ゆうべ". If you specifically want it read as "ゆうべ", you might as well write it all in hiragana from the beginning.／The other is "evening". In this case, you usually write it as "夕べ".