And it came to pass in those days that a decree went out from Caesar Augustus that all the world should be registered.
Listen to the "Verse by Verse" episode covering this scripture.
The vast Roman Empire was a structured and expensive government, and this census was “organized to facilitate the collection of taxes” (NKJV Study Bible). In an agrarian society, both tithes and taxes were collected shortly after the fall harvest when the farmer would have sold his crop. Collecting taxes in the winter or any other time is not very effective.
Also, since such a census required people like Joseph to travel back to their ancestral home, it is unlikely the census would have been conducted in the winter time when travel was difficult. This is but one more hint in the text that Jesus Christ’s birth did not occur on Dec. 25. Though the Bible does not give an exact date for His birth, it seems clear that it could not have been in the winter.
For more about why Jesus Christ’s birth could not have been Dec. 25 and how it came to be celebrated that day, see our article “Christmas: Should Christians Celebrate It?”