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 <[email protected]> 
         Andrei Zmievski <[email protected]> 

// ...

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

HTTP Expires header

http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html

It is set to negative or past value, to prevent caching of response.

Quite common usage of this header.

I think you are using session_cache_limiter before calling session_start. If argument is private or no-cache the result is setting the Expires header to the time you have mentioned. Refer to this document for more information.