What is the difference between completing something к среде vs до среды?

This comment was originally written by Shady_arc on Duolingo Russian forum.

They are close in meaning. Using к adds a flavour of there being a deadline, i.e. ко вторнику / к среде / к концу месяца are similar to "by Tuesday"/"by Wednesday"/"by the end of the month". It usually means that, e.g., the completion of a job will be checked during that day/period (e.g., you may be asked to present your work on Wednesday).

До is a preposition you generally use to say one event happens before the other, including giving the time something should be done. До среды can mean strictly before Wednesday—or, sometimes, it can include the Wednesday. When it is important, people may add включительно to avoid being misunderstood:

• То есть русский язык должен быть до десятого класса включительно?
• Я здесь пробуду до 25-го включительно.
• Например, отрезок в пятнадцать лет (между 1984-м и 1998-м годом включительно) представлен, по воле автора, всего семью рецензиями.
• До 1998 года включительно мы не могли их никому продать.