Mejor no hubieras salido hoy.

Is anyone else struggling with the translation? I think it should be 'It would have been better not to have gone out today".

I know exactly how you feel @Nasus, it took me ages to finally wrap my head around this expression. Eventually what helped me was when I realised that You’d better not … is actually You had better not … - that is, a using ‘had’ to talk about a hypothetical future (non)action.

Si así lo cree, mejor no hubiera dicho nada al respecto. - If you think so, sir, you had better not say anything about it.
Mejor no le hubiera dicho que lo haría. - Maybe I shouldn’t have said I would do it.

The first example above demonstrates this usage, i.e. suggest what not to do, whereas the second is looking at past regrets. More examples are available at