Answer:
English, like many other languages, has roots in Latin. Latin in turn has several words from Greek. For example, "gymnasium" comes from Latin "gymnasium", itself a latinization of Greek "gymnasion". Also, loanwords from Greek (and Latin) are quite common, and some words have survived. Many English words are formed by Greek words, and Greek gods have named weekdays and astral objects, though many of these use their Roman names. But most important Roman gods were copies of the Greek, just renamed. Examples are Mercury (Hermes), Venus (Aphrodite), Mars (Ares), Saturn (Greek god of the sky before Zeus), Jupiter (Zeus), Neptune (Poseidon) and Pluto, plus Saturday, after Saturn.