Solar returns are also called birthday charts. In fact, it’s from the solar return that we get the expression "many happy returns" which we say when wishing someone a happy birthday.
Your solar return is calculated for the exact date and time when the Sun returns to the place it was in when you were born. This means that your true birthday may fall on the same date you were born, or it may fall the day before or after, depending on when the Sun reaches the correct degree. For example, if you were born on September 28, 1950, when the Sun was at 05ELIB 00', your solar return would have fallen on September 27th in the years 2004, 2005, and 2006.
Many reputable astrologers believe you can affect the outcome of your solar return by being in a place other than your birthplace or your place of residence at the time of the return. Some people will go to extraordinary lengths to visit another town or country for their solar return. This is because calculating a solar return for a different location can cause certain desirable planetary aspects to appear, and certain undesirable planetary aspects to disappear, or a planet or planets can change houses if the chart is erected for a different place. However, while the ancients believed in erecting a solar return for the location in which a person is at the time of the return, people back then did not travel much, and they could not have anticipated our highly mobile lifestyles.
The solar return takes the single day of the return and uses the 24 hours that follow the return to predict the future events for that year. But, we can argue that, like progressions, we need to recalculate the solar return for the place where the person lives once they return home.
Why is the solar return any different than a progression in that respect? After all, if I went to New York in June, I wouldn’t erect my progressions there and then return to the West Coast and rely on those New York calculations. When I progress the planets in my solar returns to predict events, it has always been the solar return chart erected for my residence that has been reliable.