Learn English – “On the first of every month” vs. “every first of the month”

Strictly speaking, the two sentences mean the same thing. However, the sentence "Each apple is red" is slightly unusual, and the more natural way to express this would be "Every apple is red", or "All apples are red."

The reason is that the word each is generally used in situations where we consider the apples individually or sequentially, whereas every and all are used for generalizations. So we might say:

We spray-painted each apple red.

Here each is appropriate because every apple was painted individually. However, most people wouldn't say the following:

[?] Each apple turned red by October.

This isn't technically wrong, but it sounds unnatural. Much more usual would be to say one of the following:

Every apple turned red by October.

All the apples turned red by October.

The latter describes your salary: if you sit at your desk and don't get fired, you'll get $500. The former is more a description of the situation: somehow, by hook or by crook, I manage to scrape together $500.

The use of "per" imply a causal connection between the passage of time and the arrival of your pay. "Each" encompasses the possibility there's no connection, and it's just happenstance.