An unforgettable birthday is a birthday that leaves a lasting impression and is memorable for a long time. Some ways to describe an unforgettable birthday in Engpsh might include:
A birthday that was full of surprises: "I had an unforgettable birthday this year because my friends planned so many surprises for me."
A birthday that was spent with loved ones: "My unforgettable birthday was the one where I spent the day surrounded by my family and closest friends."
A birthday that involved a special activity or event: "I'll never forget my 21st birthday because we went skydiving to celebrate."
Overall, an unforgettable birthday is one that stands out for its meaningful or unique experiences.