Persian (Farsi) to English Art/Literary Poetry & Literature Literary Persian

Greetings,

With my great love of pure Persian, I was fascinated to see this site:
http://www.youtube.com/watch?v=mINYxN6prFs

There is a sentence of pure Persian:
زادروز اشو زرتشت پیام آور
My understand is that this means “? birthday of Zoroaster the Prophet.”

But what is this word اشو, please? What does it mean and how is it pronounced? I guess the final و must be a short O as there is no written kasre.

The one-volume Āryānpur Progressive Persian-English Dictionary actually gives this word as part of its entry on the word ‘مقدّس’, stating that Persian synonyms for the word مقدّس are اشو and سِپَنْتا (the vocalization is mine). But I find no trace of اشو as a dictionary entry in its own right.

Best wishes, and many thanks,

Simon