Happy unbirthday to you!
In astrology, the Sun represents our major personality we "shine" into the world, consciousness, identity, what energizes us, and a large part of our life purpose and part of our reason for being on Earth. This solar identity is so important we celebrate it once a year, though if we celebrated our lunar returns, which I sometimes do, it would happen once a month (more on that next time).
The natal astrology chart represents the moment we were born and the arrangement of the planets accordingly; however, throughout our lives, the planets continue to move around the sun and come into relationship with our personal chart. One of the more important transits we have are called “returns,” which are moments when a planet returns to the same place it is in our chart when we were born (i.e. the Saturn Return).
The “Solar Return” is one such return that happens once a year, often called our birthday (though I call it my “Earth birth”). This day astrologically harnesses the energy of the sun and can be recharge or re-evaluation for your life purpose and soul’s journey on Earth. It’s important to note that your birthday and Solar Return may or may not occur on the same day. This makes sense when considering the calendar year and the need for Leap Days every 4 years since the year is not really 365 days, but 365 ¼ (dang confusing Gregorian calendar). The timing of the solar return also depends on your location since the astrology chart also relies upon latitude and longitude for the degrees of the zodiac wheel.
Here it is in short: The Solar Return is a yearly transit where the degree of the sun in the sky lines up with the degree of our natal sun. Our birthdays may or may not correlate with this day as the Gregorian calendar is not based on degrees of the zodiac.
Example: My birthday is on May 20, though many years my solar return occurs on May 19. As a cusp birthday between Taurus and Gemini, my actual birthday falls when the sun moves into Gemini.
In addition, astrologers use what is called “secondary progressions” as another way of interpreting the unfolding or integration of a natal chart. This technique operates on symbolic time instead of actual time like a transit, where one day equates to one year of life and the birth chart is progressed like a transit. Then we look at major aspects of the new sun location in relationship to the natal chart for insights into specific patterns for someone's birth year ahead. Each year of life then moves the sun one degree forward.
I rarely do progressions as my main astrological modality, Archetypal Astrology, focuses more on planetary archetypes than predictive astrology and forecasting. However, I use secondary progressions a lot with my own chart and have found incredible correlations with events and major learning phases. When the progressed sun changes signs, around every 30 years, I noticed huge difference. A few years ago when my progressed Sun moved into Cancer it was right after I graduated with my Master's (when my sun was in curious Gemini for the past 29 years), and I shifted my life much more into stability and safety. I even got a cat so I had someone to nurture and care for and my usual Sun-Uranus adventurous tendencies became more mellow and home-oriented.
Our natal planets in relationship to the progressed Sun, the degree of the Sun, and the house it falls into all offer clues into how we might express ourselves over the year ahead. With progressions, we can also look at our Moon and other personal planets for insights into different realms of growth and change. Though since the Sun is often the center of focus for astrology, it makes sense to start with its position and explore from there.
Example: My 35th year on Earth and my progressed sun are two degrees Cancer in exact square with Pallas Athena and a trine with Jupiter. I've already felt a tension between my desire to go back to school though it feels out of alignment as well as I feel more expanded into my power thanks to Jupiter. It is in the 12th house which could bring up issues of feeling a bit lost at sea and not sure of my path or conversely being more in the flow with my own rhythms. With the sun in the 2nd degree of Cancer it is associated with the planet Mars and could be a year of lots of action and passion.
Ah yes, and then there is another way of mapping imaginary time and cycles called an annual profection. It's a yearly advancement through the chart by house based on the age. Beginning with the Ascendent or 1st house and moving through and around to each sign. During that year of life that house and the planetary bodies in it would be activated. This technique is associated with more traditional astrology but I find it useful for understanding subtle themes by year.
Want to find out more or learn more about your Sun beyond its sign such as solar return and progressions? Well, of course, seeing an astrologer is an excellent option for perfections and progressions, though if you have astrology software you can look for yourself the day and time of when the Sun in the sky returns to the same degree and second as when you were born.