What is the difference between congratulation and congratulations?
congratulation |kənˌgra ch əˈlā sh ən; -ˌgrajə-| noun an expression of praise for an achievement or good wishes on a special occasion; the act of congratulating : he began pumping the hand of his son in congratulation. • ( congratulations) words expressing congratulation : our congratulations to the winners | [as exclam. ] congratulations on a job well done!
You make the call.
It is difficult to articulate the difference between them.
When you are speaking about the act of praise, a single act of praise is a congratulation, multiple is -s. You can almost always use the plural form and sound perfectly normal.
When you wish to praise someone:
I congratulate you. (much more formal, can easily sound distant or impersonal)
You have my congratulations. Shortened: Congratulations! or Congratulations on your wedding! (always plural in shortened forms seemingly expressing multiple praises, explanation for what the congratulation is for only necessary when it isn't clear)
The difference is very subtle :
"congratulation" is congratulating, that is having some joy, some pleasure, perhaps by yourself and even secretly, for instance because you have won at the lottery ;
congratulations" are the expression of that, for instance to a newly married couple.
The word comes from Latin congratulatio, with exactly the same meaning.