According to scholars the birth of Jesus is controversial. Most would consider it to be September 17th, 07 BC.

But in what year was it ordered that all people were to celebrate December 25th as the "correct" date of Christ’s birth?

AD .356 by Pope Julius 1
http://www.sundaytimes.lk/031214/plus/5.htm

My answer differs slightly from yours Jenny.

I first heard it on the radio and then found a web site to confirm it.

(2) two years prior to julius I (end of his reign) in 354 , bishop Liberius was the one who ordered it. Source

What I meant is that Julius I died in 352 Source so the year 356 that your source mentions is wrong. ( I was too late to edit)

I found another site that dates it to AD.350 also by Pope Julius. I don't think people really know for sure do they?
here

Humm! I don't see AD.350 on your source. It is mentionned that the pope Julius I chose december 25th. as the date to be celebrated.

I think that he might have chosen the date ( December 25th) back in AD.350-352 or so but it was "enforced" in 354 AD by Bishop Liberius,... he made it official. ( These two guys must have been good friends anyway)

It would make sense this way.