I've read/heard conflicting opinions on whether it was the Emperor Constantine or what later became known as the Catholic Church that decreed the holidays of Christianity and the status of Jesus (whether man or God) at the Council of Nicea. Which was it?

by fromRonnie
talondearg

Neither, because neither of those issues were tackled at the Council of Nicaea.

The Council of Nicaea was initiated at the behest of Emperor Constantine to deal with what was essentially becoming an empire-wide split in the church over the issue of Arius and his theology. Arius did indeed teach that Jesus was a created being, who was a 'second' after God the Almighty, and so a distinct and separate entity. Nonetheless, Arius would vehemently insist that Jesus was 'god', he just didn't mean "god-God".

It's important to realise this because prior to this whole period the definition of what counts as 'god' and what counts as 'creation' appears not always as distinct as it does afterwards. Part of the reason Arius' position made sense is precisely that he did consider Jesus 'god', just not 'god' like God is god.

At no point did the Council debate whether Jesus was (a) a man versus (b) God. They were deliberating about what it meant that Jesus was God.

Secondly, Nicaea is not really convened to deal with christian holidays. Constantine did appear to have an aim of dealing with the ongoing issue known as "Quartodecimanism", which was an original practice of celebrating Pascha (easter) according to the Jewish calendar, and so on the 14th Nisan on whatever day of the week that happened to fall, rather than on first Sunday following that date (which was the Roman practice and became the dominant one).

The council did not deal with Christmas at all.

You can easily read the canons of the Council for yourself, which also puts to rest several other common misconceptions.

On the Quartodeciman issue, DelCogliano has written persuasively on that issue recently.
DelCogliano, Mark (2011). "The promotion of the Constantinian agenda in Eusebius of Caesarea's On the feast of Pascha". In Inowlocki, Sabrina; Zamagni, Claudio (eds.). Reconsidering Eusebius: collected papers on literary, historical, and theological issues. Supplements to Vigiliae Christianae. 107. Brill.

For an in-depth background to the period and the issues, I draw upon:
Lewis Ayres, Nicaea and its Legacy (2004)
John Behr, The Road to Nicaea and The Nicene Faith
Khaled Anatolios, Retrieving Nicaea