Why the Day of the Week You Were Born Feeds Into Your Compatibility Score
Ask for someone's birthday and you already have their day of the week — no birth time, no guesswork. That's exactly why it's one of the few genuinely deterministic inputs Synastri's `/compat` score can use: a fixed, ancient correspondence between the 7 days and 7 classical "planets" (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn), same order every week, going back to the Hellenistic calendar.
One planet rules each day, in a fixed order
Sunday is the Sun's day, Monday the Moon's, Tuesday Mars's, Wednesday Mercury's, Thursday Jupiter's, Friday Venus's, Saturday Saturn's — the same Sun-Moon-Mars-Mercury-Jupiter-Venus-Saturn sequence several modern languages still preserve in their weekday names. Synastri reads this straight off `Date.getDay()` for a birthdate, so it needs nothing beyond the date already on the form.
Some day-pairs are read as harmonious, some as tense
The system checks the two ruling planets against a short classical list: Sun paired with Moon, Venus paired with Mars, and Jupiter paired with the Sun are read as harmonious (a small positive nudge); Saturn paired with Mars and Mercury paired with Moon are read as tense (a small negative nudge). Every other combination — most of them — is neutral. It's a light touch on purpose: this single factor moves the score by only a few points either way, never enough to flip a strong match into a weak one on its own.
Why bother with something this small
Because it costs nothing to check and it's a real, disclosed piece of the traditional system rather than filler — the kind of detail a hand-cast astrology reading would use too, just automated here instead of looked up in a table. It folds quietly into your `/compat` score alongside the bigger sun-sign and element factors; check your own on `/compat` to see it in the mix. For entertainment.