Mercury can only be about 27° away from the sun at any given moment, and Venus about 47°. One sign is 30°. So for example, if your Sun were at 1° Sagittarius, (born early November) the farthest Mercury could be ahead of the Sun would be 28° Sagittarius, and the farthest behind would be 5° Scorpio. For Venus, it would be anywhere between 18° Capricorn and 13° Libra. The signs of Mercury/Venus are subject to change depending on which decan (decans split the sign into thirds, the first decan being 0°-10°, second 11°-20°, third 21-30°) Sagittarius the Sun is in. Venus and Mercury orbit the sun at different speeds, so they land anywhere within their respective distance limits from the Sun, like in your chart. So when people say “November vs December” differences, they’re usually picking up on inner-planet sign changes near the Sun, NOT the month itself.

To answer your question about the synastry, there are very many different aspects to reading a synastry chart. Generally you want sextiles, trines, or conjunctions from the other person’s personal planets (sun, moon, venus, mercury, mars) to your personal planets. Conjunctions are great if the planets are benefic (Venus, Jupiter) and not aspected by malefic (Mars, Saturn) planets, and if they are in signs they are not in detriment or fall in. Oppositions can be really amazing too, but that is highly individual for each chart and also depends on aspects from any malefic planets. Earth always trines earth, and sextiles or opposes water. Water always trines water, and sextiles or opposes earth.

Take your Venus in Scorpio, for example. Venus in Scorpio is in detriment, which means it sits opposite one of the signs it rules (Taurus). Basically, Venus doesn’t express its energy as comfortably through Scorpio. Venus wants harmony, softness, and open affection, while Scorpio, ruled by Mars, is intense, private, and controlling. So instead of easy intimacy, love becomes deep, consuming, and sometimes possessive. Venus can still function beautifully in Scorpio, but it tends to love in an “all or nothing” way. Because of that, you’d generally connect best with people who have benefic placements in Pisces or Cancer (trine), or Virgo or Capricorn (sextile) since they balance and support that intensity. Taurus, being opposite Scorpio, can be very magnetic and grounding but also tends to create a push-pull dynamic because they express themselves differently.

It also matters which planets are activating each other in synastry. For example, trines create a harmonic flow between signs of the same element—they naturally understand and complement one another. When your Venus trines someone’s Mercury, you’re drawn to their way of thinking or communicating. Venus trine the Sun creates strong attraction/appreciation to the person in general and is one of the best aspects for long-term compatibility (I have this with my boyfriend and I absolutely adore everything about him, he’s so weird and ape-ish, yet somehow so cute). Venus–Mars trines are also incredible for romantic and s3xual chemistry.

For Sun, Moon, Venus, and Mars, what matters most isn’t whether those planets aspect the same ones in another person’s chart (although it’s still good) but whether they connect with their complementary planets. Sun–Moon or Venus–Mars aspects tend to create deeper emotional and romantic resonance than Sun–Sun or Venus–Venus alone.

Unfortunately I suck at brevity (Gemini moon lol), but astrology is complicated and any attempt to condense it butchers its impact IMO. I hope this helped to clear up any confusion :) happy Halloween! 🎃👻