Ancient astrologers did not look to favorably upon the planet returning to the same sign as that of birth. However, they primary had in mind the ingress through the entire sign while the modern use of return charts may be more specific. In modern astrology, the solar return chart is calculated when the Sun returns to the exact degree and minute of the Sun in the natal chart. This occurs around one's birthday. The affects of a Solar Return chart last for about one year.
Some astrologers claim the chart can take hold a few months prior to the birthday.
The location used for the solar return chart is in dispute among astrologers. Some prefer to use the same location for the birth chart. Others calculate the chart using the primary living location for that year. Still others claim that the location where a person has traveled to on the birthday should be used for the chart for the entire year. Having heard testimony by persons who have traveled on their birthdays for a better yearly chart, I don't think this method has much value.