1)
What is the meaning of "Yaladti"?
1.
Rashi: It is conception. Yirmeyah's father had Bi'ah with his wife amidst Ones, on the day he fled Menasheh, who was killing the Nevi'im.
2.
Radak: It is the day he was born. "Asher Yeladasni Imi" repeats the matter in different words.
2)
Why did he curse the day?
1.
Radak: Tzadikim, amidst bitterness, curse the day they were born, even though the curse will not take effect, for it already passed. Also Iyov (3:3) said "Yovad Yom Ivaled Bo
1
Even before the Churban, it was the day that Hashem decreed that Dor ha'Midbar will die in the Midbar (Ta'anis 4:6)! (PF)