How to decide whether to read 「昨夜」 as 「さくや」 or「 ゆうべ」?

Question

I found an answer saying that 「さくや」is the correct reading of 「昨夜」, but despite that I often encounter it as「ゆうべ」.

Is there a difference in meaning or are both readings interchangeable?

Answer 1

Both are correct readings. It depends a lot on the context, but ゆうべ is informal and さくや is formal.

The same can be seen in 明日 with あす and あした, or in 昨日 with きのう and さくじつ.