As per my interpretation of the message of the Bhagwad Geeta, the soul is eternal. The birth and death of the body are incidental and material. If this applies for mortals, then why does the birth of a deity considered a special occasion? Shouldn't it be considered an ordinary event?
The answer is in your question itself
"The birth and death of the body are incidental and material. If this applies to mortals.."
Yes, it applies to mortals.
"..then why does the birth of a deity considered a special occasion?
Because it does not apply to deities.
Krishna says so himself in BG 4.9:
जन्म कर्म च मे दिव्यमेवं यो वेत्ति तत्त्वतः।
त्यक्त्वा देहं पुनर्जन्म नैति मामेति सोऽर्जुन ।।
janma karma ca me divyam evaḿ yo vetti tattvataḥ
tyaktvā dehaḿ punar janma naiti mām eti so arjuna
My birth and deeds are divine. One who knows this, leaves his body, is not born again and attains me Arjuna.
Bhagavan is celebrated as Aja and Vija - He is the unborn, yet he takes birth. Both God and soul are 'unborn' in the sense that they are not created by anyone. But souls are born in the sense that they take birth due to past karma. God is born in the sense that he takes birth due to karuna (pity), but unborn in the sense that he is unaffected by past karma, and is born of his own free will.
The reason Krishna is born is to prevent us from being born again. But being adamant, we want to enjoy & suffer in this playground of samsara.
We celebrate Janmashtami to remind ourselves that Krishna is waiting for us to join him.
To answer the title question
Why is Janmashtami an important festival in Hinduism?
Krishna is associated with absolute joy or bliss. Hence, it serves as a reminder for us to move towards bliss and also to spread joy in the society:
Janmashtami is the day when we celebrate the birth of Lord Krishna. Lord Krishna signifies Ananda (bliss, pure happiness). We express any happiness by celebration. So Janmashtami is celebrating the birth of Bliss. It is the day when that (divine) joy became manifest. Actually, it is not right to use the word ‘birth’ here. Lord Krishna was never born. That (divine) joy was always present, but it is on this day that it became manifest and evident (in the form of Lord Krishna). So where should you look for Lord Krishna? Everywhere! Lord Krishna says – ‘One who sees Me everywhere, in everyone, and sees everyone in Me is truly intelligent’. Now, no one wants to be foolish in this world. Everyone wants to be intelligent. So, one who recognizes the Parmatma (Supreme soul or all-pervading one Consciousness) as that Anand-tattva (the element of Bliss) that is manifest throughout Creation is truly intelligent. Everything has emerged from that Anand-tattva. This is the special message of Janmashtami. What does one need for this? You need to be innocent at heart; you should have that child-like innocence. It is said – ‘Bhole bhaav miley Raghurai’ (One attains the Lord through the innocence and purity of feelings).
Today’s knowledge is this: It’s time to seriously enjoy and celebrate! (Laughter) You can realize Lord Krishna only through deep rest. You cannot realize Lord Krishna through your struggle and efforts. And when can you get that deep rest? Deep rest happens when your mind is not restless and entangled with feverish desires; when it is not caught up in cravings and aversions. It is in those moments of deep rest that one witnesses the birth of Lord Krishna (here meaning divine bliss)...
Excerpts from an article by Sri Sri Ravi Shankar: http://www.artofliving.org/wisdom/sri-sri-message-janmashtami