Can anyone here please tell me the difference between 'way back then' and 'way back when' ?
Both informal phrases are used as a phrase of time comparison. The usage differs though.
Way back then:
"Ah, those good ol' days. The 1960s. Sigh. Way back then, the skies were clearer and the people were kind."
Way back when:
"Do you remember the day we first met? Twas that sunny day, way back when you still used to wear those horn-rimmed glasses!"
In the former case, the "then" shows that the time period has been mentioned before, unlike in the case with the latter, where the "when" specifies the time with some incident.
Way back then - indicates something that happened in the past, but the time is usually specified in some previous instance. So it's like an additional form of wording to the previous already stated time.
Way back when - refers to something that happened in the past, the time is not specified here by previous instances, and the word "when" symbolizes an event or usance that helps us determine the time or at least help us with our perception.