What is the difference between :
Has been postponed and was postponed?

Thank you

Question

What is the difference between :
Has been postponed and was postponed?

Thank you

Answer

You would say "..has been postponed' once it has been decided to postpone it, but before it happens.
Say "was postponed" after it happens.

Answer

"Has been" postponed is the present perfect tense.  "Was postponed" is the simple past tense.  
Study the verb tenses:  https://www.englishpage.com/verbpage/verbtenseintro.html