"Before date" versus "by date"

There is a difference, yes.

If you want the job to be ready on Tuesday morning, you would say

Please do this before Tuesday

If you need it to be done either before or during Tuesday, you would say

Please do this by Tuesday

In other words, using by is inclusive, it means do this on any day up to and including the day specified. Using before is non inclusive, it means that I expect it to be done when I arrive on Tuesday morning.

If you want to be precise and want it done literally before a certain time, then "before" is the the word to use. If you want it done on or before the specified day or time, the "by" is the right word.

A couple of other expressions that are used (especially in business) are "not later than" (abbreviated NLT)) and "by close of business" (abbreviated COB).

Although, by definition, “before Tuesday” actually means “by Monday at the latest”, many people still confuse “by” (up to AND including) and “before” (up to BUT excluding).

So, in reply to the initial question, it is NOT incorrect to say "Please do this before Tuesday"... but if you mean it can still be done on Tuesday, you should definitely rephrase it as "Please do this BY Tuesday".

Also, even if you DO mean “before Tuesday” (i.e. NOT after Monday), you should still consider rephrasing it as “by Monday”, for the sake of clarity, because unaware people might interpret it as “by” even when you really mean “before”.

Just keep in mind that most people don't take the time to check in dictionaries when they try to understand instructions. They just go by instinct, so you might as well make sure there is no possible ambiguity (whatever dictionaries or linguists might say).