Why Does the Hijri Anniversary Change?
The Islamic (Hijri) calendar is purely lunar, based on the cycles of the moon. A lunar year is approximately 354 or 355 days long, compared to the 365 days of the solar Gregorian calendar. Because the Hijri year is about 11 days shorter, your Hijri anniversary will drift backward through the Gregorian calendar. If you got married in the summer, your Hijri anniversary will eventually fall in the spring, then winter!
The Sunnah of Expressing Love
The Prophet Muhammad ﷺ was the most loving to his wives. He would race with Aisha (RA), drink from the exact spot on the cup where she drank, and openly declare his love for Khadijah (RA) even years after her passing. Using your Hijri anniversary as a reminder to express love, give a gift (Hadiya), and make Dua for your spouse is highly encouraged.