Everyone knows the rule (not universally followed) that when a word ends with the letter S, we just add an apostrophe right after the word, instead of adding an apostrophe and an S after it. So, according to the above rule of thumb, it's
The noun itself is pronounced fully with an S at the end, and then the S sound is pronounced immediately.
But there's another category of words that end with an S - i.e. most plurals. In this case, at least in writing, we (again, not universally) follow the same rule:
April Fools' Day
But I have noted that in the second case, the missing S after the apostrophe is not pronounced at all, even in formal speech.
Is this normal?