The Western tropical zodiac defines spring as the quarter of the year running from the March equinox, when the sun crosses the celestial equator moving northward and day and night stand in approximately equal length worldwide, to the June solstice. Astrologically the season opens with Aries, the first sign of the zodiac and, in the tropical system specifically, the sign whose beginning is defined by the equinox itself rather than by any fixed star position, followed by Taurus and then Gemini as the sun continues its apparent path along the ecliptic through the season. Each of the zodiac's four seasons opens with what astrology calls a cardinal sign, Aries for spring, initiating a new quarter's energy, moves through a fixed sign, Taurus, that stabilizes and consolidates it, and closes with a mutable sign, Gemini, that adapts and prepares the transition into the next season, a threefold modality pattern the tropical zodiac repeats identically in every one of its four seasonal quarters.
Because the tropical zodiac is defined by the equinoxes and solstices themselves rather than by the constellations' actual current positions, which have drifted against the tropical sign boundaries over roughly two millennia due to the precession of the equinoxes, a birth chart's placement of the sun in tropical Aries reflects the sun's position relative to the March equinox point specifically, a distinction from the sidereal zodiac systems, used in Vedic astrology among others, that track the visible constellations directly.
Sources TetrabiblosClaudius Ptolemy with A History of Western AstrologyNicholas Campion