It's not obligatory but there are hadiths that attach a very high Islamic significance to it and the fact that Zoroastrians celebrated it doesn’t discredit its sound religious basis. We know this owing to the perennial wisdom possessed by the Shia Imams (who are in fact the inheritors of the Prophetic wisdom) who revealed to us the metaphysical and religious significance of this important occasion that has been celebrated by Zoroastrians and probably by other ancient Divine religions since time immemorial. Hadiths also recommend particular Islamic rituals on this day such as ghusl, perfume, fasting and prayers. Based on the hadiths this sacred occasion came to be neglected and forgotten by Arabs of Jahiliyya (Arabs living before Islam) and Sunni Muslims alike. This ignorance persists to this day as in the widespread assumtion that Shia celebrate a “majus” (a derogatory word for Zorastrians) ritual. The significance of Nowruz can also be shown in the light of Quranic verses that consider the renewal of Earth as a sign of Allah's power and an allegory for Resurrection.
Nowruz in hadiths
Here are some narrations disclosing the religious significance of Nowruz by Shia sources:
Sheikh Tusi (http://en.wikipedia.org/wiki/Shaykh_Tusi) narrates a hadith in his Misbah Al-Mutehajjid from Mu’alla bin Khanees, from Imam Jafar al-Sadiq, the 6th Shia Imam who said:
Nowruz is the day when the Qaim (the riser, al-Mahdi) from among us Ahl al-Bayt and the possessor of the command rises, and Allah makes him victorious against Dajjal. He will hang Dajjal in a wasteland in Kufa (an important city in Iraq). And there’s no Nowruz except we expect the opening (the rise of Mahdi) on it as Nowruz is one of our days. The Persians preserved it but you Arabs spoiled it!
Mu’alla has also narrated from Imam Sadiq that
I entered Aba Abd Allah (a title for Imam Sadiq) on the morning of a Nowruz and he said: “O Mu’alla! Do you know what day is this?” I said “No! But it’s the day that Persians celebrate and congratulate each other”. He said: “No! By Ka’ba in Mecca, this day is not but for an archaic matter and that I will reveal to you.” I said: “A knowledge like this granted to me by your Holiness is more desirable to me than if I could live an eternal life. May Allah destroy your enemies.” He said: “O Mu’alla! Nowruz is the day that Allah put an oath on His servants that they shall worship Him and not associate Him partners, and follow the religion of His messengers, proofs and friends, and it is the day that Sun first rose, and the fertilizing winds breezed and the blossoms of the Earth grew, and it is the day when the Ark of Noah subsided on the Judi Mount, and the day Allah brought back to life thousands of a people who had left their homes out of fear. Allah told them to die and they died and He then brought them back to life. It’s the day that Gabriel descended on the Prophet (pbuh), and the day Abraham shattered the idols of his people, and the day the Messenger of Allah had Ali (pbuh) mount his shoulders so that he throw and shatter the idols of Quraish from the top of the Kabba.
In Muhadhab, the same narrator narrates that Imam Sadiq (a.s.) said:
Nowruz is the same day that the Prophet got baya’t (allegiance) for Amir Al-Mu’mineen (as — meaning Ali) on the day of Ghadir and everyone acknowledged his wilayah and those who remained loyal to it are blessed and those who broke it are doomed and it is also the same day the messenger of Allah (saw) sent Ali to the valley of jinn to get their pact and the day of his victory over the people of Nahrawaan and the day he killed Dhu-Thadiyyah. It is the day our Qaa’em (The Awaited One) of our progeny will emerge with his companions and may God defeat the Dajjal with his hands and hang him on the synagogues of Kufah.
There are other hadiths along those lines from Prophet Muhammad (pbuh) and Imam Ali ibn Abi Talib (pbuh). So Nowruz is an ancient Persian but originally a Divine Islamic occasion.
Nowruz in the light of Quran
Nowruz is celebration of renewal of life, fertility and beauty in nature. For this reason the occasion can be viewed from an Islamic perspective, i.e. witnessing the renewal of Allah's creative power and His beautiful names in spring as well as an example of renewal of life in Qiyama. This was partially mentioned in the second hadith above, but here are some Quranic parallels:
And it is He who sends down rain from the sky, and We produce thereby the growth of all things. We produce from it greenery from which We produce grains arranged in layers. And from the palm trees - of its emerging fruit are clusters hanging low. And [We produce] gardens of grapevines and olives and pomegranates, similar yet varied. Look at [each of] its fruit when it yields and [at] its ripening. Indeed in that are signs for a people who believe. (6:99)
And it is He who sends the winds as good tidings before His mercy until, when they have carried heavy rainclouds, We drive them to a dead land and We send down rain therein and bring forth thereby [some] of all the fruits. Thus will We bring forth the dead; perhaps you may be reminded. (7:57)
And you see the earth barren, but when We send down upon it rain, it quivers and swells and grows [something] of every beautiful kind. (5) That is because Allah is the Truth and because He gives life to the dead and because He is over all things competent (6)
Now all of these may suggest that the despised "Majoos", at least in this respect, were much more observant and cognizant of Divine signs than do many self-righteous Muslims! So let's acknowledge that all world religions contain elements of truth apart from partial decadence that affects all religions through time! Indeed the Shia theosophy has shown that Zoroastrianism as a religion is remarkable for containing traces of super-historical perennial wisdom that unites all Divine religions. This was first shown by Shahab al-Din Suhrawardi who founded the school of “Illumination” or “Ishraq” in Islamic philosophy. Suhrawardi was partly influenced by the Zoroastrian theosophic tradition called “Pahlavis theosophy” many of its doctrines are paralleled in Quran such as the allegories of light and darkness for referring to God and evil and how truth which consists in Divine theophanies is realized via internal illumination.