Translation:It is not so important to do it today.
Is is mandatory to include the second "es"?
Can one say: "Es ist nicht so wichtig, heute zu tun"?
That would be as wrong as "We don't have to do today" -- the "it" is missing.
In German, the first es is not really "it" -- it's a dummy subject, present only because something has to stand before the verb (otherwise the verb wouldn't be in the second position). The real subject of ist nicht so wichtig is the phrase es heute zu tun, as if it were "Doing it today is not so important", where again, you cannot abbreviate that to "Doing today is not so important".