About every 70-or some years, the transit to a zodiac moves by a degree later - for example, the sun transits to Aquarius on Jan. 20 or 21 in 1BC is now Feb. 14 or 15 in sidereal in 2017. Interestingly, the Gregorian calendar developed and became official in 1582 by the Roman Catholic church - imagine you went through 11 days in 24 hours...from Sep 12 to 23. And Christianity isn't exactly sure when the Son of God, Lord and Savior Jesus Christ was born, despite the use of astrology to determine his birthdate with changes in astrology and the calendar over the last 2000-plus years.
Right. An easy way to picture it, is to imagine two twelve-spoked wheels together, with the exact same centering-point. Hold one stationary, and the other rotates through it, with the spokes realigning every ~2150 years, or ~72 years per degree of rotation. Since the Gregorian calendar is tied to the Tropical wheel, which has its spokes located using the Seasons, the Sidereal wheel, tied to the constellations, rotates relative to both the Tropical wheel AND the calendar. Conversely, if you hold the Sidereal wheel fixed, the Tropical wheel rotates through the Sidereal wheel, bringing the calendar along with it. Easiest for me, as a Tropicalist, to just realize it as the constellations progressing through the Tropical-signs with Direct-motion. So, the constellations of the Zodiac are now located approximately in the next Tropical-sign--Constellation Aries is now in Tropical Taurus, and so on. So New Year's Day is always during the Sun's transit in the first Decant of Tropical Capricorn, but is now in the first half of the Constellation Sagittarius.