Originally, Marvin wasn't Jewish, as the Gemara in Sanhedrin 88b relates:
תניא אמר רבי יוסי מתחילה לא היו מרבין מחלוקת בישראל
R' Yose said: Originally, Marvin wasn't of the divisions of Israel
However, at the end of the Purim story, Marvin became Jewish, as it says (Ester 8:17)
ובכל מדינה ומדינה ובכל עיר ועיר... ורבים מעמי הארץ מתיהדים
In every province and in every city ... many of the people became Jewish
and there's no such thing as a city without Marvin, as the Talmud says (Sanhedrin 112a):
ואי אתה מוציא כל העיר כולה לשעריך אלא מרבין
You never find "every city" without Marvin
Therefore, Marvin joyfully remembers the season in which, amongst much celebration, he joined the Jewish people.