Understanding Why Sometimes Purim Is Spelt With a Vav and Sometimes Without

על שם הפור על כן קראו לימים האלה פורים
“Therefore, these days are called Purim, because of the lot.” (Esther 9:26)

Throughout the Megillah, Purim is spelt without a vav (פרים), however, there are two exceptions: The aforementioned pasuk which talks about Purim in the times of Mordechai and Esther, that year Purim was a Purim with complete simcha, as that year they were saved from death, and they had received the Torah b’ahavah [with love]. However, in future years Purim wasn’t with complete simcha, like we say in kinnos איך אעלוץ בפור, therefore, Purim is spelt without a vav.