No one knows for sure just how singing Handel's oratorio became such a family tradition. Certainly, my Grandma and Grandpa Thompson loved singing the "Hallelujah Chorus." Grandma Lora had sung in the Mormon Tabernacle Choir when that great chorus had become synonymous with the choir in the early days of Music and the Spoken Word. When four of their children later sang in the Tabernacle choir, they already knew and loved Handel's Messiah.
I remember big family gatherings for Thanksgiving, Christmas, or Easter when the main event (after the meal) was singing. My aunts and uncles loved to sing and they particularly loved singing Messiah together. They sang the choruses, of course, but also sang the solos. Nearly all of them had sung as soloists professionally. It seemed that whenever Messiah was performed locally, they were sought after for solo work. These family sings went on for hours. (It takes that long to sing a major oratorio.) It was tricky crowding all of the singing relatives into the favorite gathering place, my Aunt's small house. (I thought the house would explode from all the people and their very loud singing!)
We love singing Handel's Messiah together! Now it can be Christmas.