The word "Labarum" refers to a specific use of the Chi Rho, and usually designates a Chi Rho used as part of a standard or insignia.
The Labarum comes to us as the possible result of a wonderful legend about Emperor Constantine. The story goes that In the year 312, the Emperor Constantine went to battle with an opponent rumored to use magic to gain victory. Constantine decided to try praying to the god of the Christians. Constantine, after praying, saw a vision – a Chi rho, and heard the words "by this shall you conquer." Constantine was victorious, and converted to Christianity. He eventually began wearing a Chi Rho on his helmet as shown below.
You are far more likely to find the "descendants" of this image than the image itself. You could argue that Chi Rho's and Celtic Crosses are both the result of the Labarum. Otherwise, most images of the Labarum will accompany art featuring Constantine or Roman soldiers.
Begin by reading the abbreviated version of Constantine's conversion above.
Read Acts 9.1-19 with the conversion story in mind. Compare and contrast the stories.
Do you know anyone who has had a similar experience? Have you had one yourself?
Some historians doubt that Constantine's conversion happened so quickly. Others have questioned whether it was genuine. What do you think?
The Labarum is difficult to preach since it is so rarely used. The conversion of Constantine might make for an interesting story to use when highlighting a conversion like Saul's in Acts 9. Ultimately, I am inclined to believe that you're better off telling Saul's story on its own, but the Labarum could wind up being a visual reminder of Saul's story.
The Labarum doesn't fit neatly in any particular area, so it can be used as easily during ordinary time as during any other part of the liturgical calendar.
Ultimately, this is a symbol that may be more helpful during an in depth study rather than during worship. The fact that your worship space does not likely have a Labarum further accentuates that proclivity.