Why is “Expires” 1981?

It’s an attempt to disable caching.

The date is the birthday of the developer Sascha Schumann who added the code.

From session.c:

Authors: Sascha Schumann <sascha@schumann.cx> 
         Andrei Zmievski <andrei@php.net> 

// ...

CACHE_LIMITER_FUNC(private)
{
    ADD_HEADER("Expires: Thu, 19 Nov 1981 08:52:00 GMT");
    CACHE_LIMITER(private_no_expire)(TSRMLS_C);
}