Hi Slava In general, I agree with Ivan's answer (above), but just to add that "celebrate" is more often used for happy events, whereas sometimes "mark" means to 'recognise the importance of'' an occasion. For example: "Next year it will be 10 years since the terrible loss of life in the earthquake , and authorities are already planning to mark the occasion." - "celebrate" is not quite the right word in this context Hope this helps Chris

Hi, Slava. To "mark an occasion" means to "celebrate something" For example, "My grandparents will have been married for 50 years next March. Make sure you mark the occasion; we're going to throw a HUGE party."