What is the difference between an Earliest Known Use cover and a First Day of Issue cover?

We're all familiar with First Day Covers. The USPS announces when a stamp will be released. Until a few years ago, the first day of sale was at a single town or city. Today, new issues are put on sale nationwide although the USPS usually still designates a town or city as the official site.

Earliest known usages come in two flavors.

Before 1920, US stamps were not issued with an official First Day of Issue. A few stamps like the 1893 Columbians and others were released on a specific date that was publicly known. But in most cases, the USPOD had the BEP print stamps for them and they released them as needed. So the Columbians and a few other issues are well known with First Day cancels. But for most pre-1920 stamps, all that can be determined is the Earliest Known Use by the postmark.

After 1920, the USPOD started officially issuing stamps on First Day date. Collectors could prepare covers for the official postmark. However, many of the modern issues were released ahead of time by local postmasters at other post offices. Just about every modern issue is known with a cancel that is prior to the official First Day of Issue. These modern Earliest Known Use covers are interesting, but of nominal value usually a few dollars.