What is a Jewish Birthday?

One's Jewish birthday is, of course, the date upon which one was born according to the Jewish calendar.

Far from considering it incidental, Jewish tradition regards one's Jewish birthday to be brimming with meaning and relevance and, in some ways, even a mini-Rosh Hashanah!

The Talmud informs us that on our Jewish birthdays our mazel (good fortune) is dominant.

The Jewish birthday is the perfect day for reflection about our lives as Jews and is an auspicious time to make new resolutions to perform good deeds and to deepen our commitment to Torah and the role it plays in our lives.

On one's Jewish birthday it is customary to get together with family and friends to celebrate Jewishly. At the celebration one should say a prayer of thanks to G‑d, give money to charity, and learn some Torah.

Establishing a "Jewish calendar" was the first mitzvah (commandment) the Jewish nation received from G‑d. This unique calendar is based on the lunar month, but is occasionally adjusted so that it remains synchronized with the solar year and its seasons.

Thus from year to year, a date on the Jewish calendar will fluctuate with respect to other calendar systems, but will always remain in close proximity to its corresponding date on the commonly used Gregorian (solar-based) calendar. For example, if your civil birthday is on June 15th, your Jewish birthday will always be within a few weeks of that date.

Your Jewish birthday has dual significance: a) According to Jewish tradition, your mazal (good fortune) is dominant on your birthday. b) As a nation we celebrate those dates when special events that affected our destiny occurred, a.k.a. holidays. As individuals we celebrate those dates that have personal significance—and what is more significant than your birth? It is when the Creator said, "Here, I am giving you a body, a soul, and a divine mission. I have absolute trust in your ability to pull through for Me."

In 1988, the Rebbe, Rabbi Menachem Mendel Schneerson, of righteous memory, inaugurated a "Jewish Birthday Campaign." He asked that we all utilize this most special day of our lives to its utmost. A day to recommit to the mission that G‑d entrusted to us—bettering and sanctifying ourselves and the world around us.

Sources: 
https://www.chabad.org/library/article_cdo/aid/144345/jewish/What-is-a-Jewish-Birthday.htm, https://www.chabad.org/library/article_cdo/aid/654993/jewish/What-Is-a-Jewish-Birthday.htm