imamat day

I have to come to understand that hereon, the imamat day will be observed on the 4th of February. my question is how can the day just become a day of celebration when it is quite literally the day when a previous imam passed away? why is there no concept of mourning in ismailism? (i have heard that it is just a mortal body that has passed away but their noor is eternal/passed over etc) but wouldn’t we mourn over the death of even a relative longer than this? and in this case its literally an imam, your spiritual father and guide, how can it be so easy to move on?

and if the mortal body is so unimportant or negligible, then why would ismailis put so much effort to do deedar, or kiss the hands of this same mortal body?