Signs, seasons and the solar year
The sidereal signs do not line up with the seasons, because the equinox point has drifted since the zodiac was defined. The tropical signs are tied to the seasons instead.
What it is
The Sun enters each sidereal sign about once a month, and the entry into Makara is Makar Sankranti. The solar year and the sign cycle are both close to 365 days, so the two cycles stay roughly together.
How it is worked out or read
Because of precession, a sidereal sign now lies about one sign behind the tropical sign of the same name. This is the reason for the ayanamsa correction.
Where it appears on this site
See sidereal vs tropical and sankranti explained.
The six ritus
The Indian year is traditionally divided into six seasons, or ritus, each of about two months: Vasanta, spring; Grishma, summer; Varsha, the rains; Sharad, autumn; Hemanta, the cool season; and Shishira, the late winter. In different traditions the ritus are tied either to pairs of lunar months, beginning with Chaitra and Vaishakha for Vasanta, or to the Sun's passage through pairs of signs.
What the drift means for the ritus
Where the ritus are counted from the Sun's sidereal signs, they drift slowly against the actual seasons, by about one day every 72 years, just as Makar Sankranti has drifted from the winter solstice. Where they are counted from the solstices and equinoxes, they stay with the weather. This is why almanacs sometimes give different dates for the start of a ritu, and why a reader should check which basis an almanac uses before comparing.
Seasons in the panchang
Many almanacs print the current ritu alongside the month and the ayana, and the ritu is named in the traditional sankalpa recited before rituals. Knowing which basis the almanac uses for the ritu helps a reader understand why the named season may not match the weather outside exactly.
In short
Signs follow the stars; seasons follow the Sun's tilt.
Why the seasons and signs drifted apart
The sidereal zodiac was fixed against the stars, so its signs would have matched the seasons in the period when the zodiac was defined. Because of precession, the equinox point has since moved backward by roughly one sign, and the sidereal Aries now begins a month or so after the spring equinox. A reader who expects the sun sign to match the season will therefore see a mismatch that is simply the result of precession, not an error in the chart.
What the Sun does through the year
The Sun enters each sidereal sign about once a month, and the entry into Makara is the basis of Makar Sankranti in mid January. The cycle of the Sun through the twelve signs is almost exactly a solar year, which is why the sankranti dates stay close to the same time of year. The shift of the signs against the seasons is a separate matter, and it accumulates slowly.