It’s that time of year again: T he lights are strung, the pines are dying indoors under the weight of shiny balls , the Starbucks mint mocha is flowing, and Mariah Carey is on heavy rotation. No matter what is happening in her career — the tribulations, the tours, the headline-grabbing performance mishaps — Ms. Carey is wildly relevant for at least one of every 12 months, thanks to the enduring popularity of her holiday recordings, particularly “All I Want for Christmas Is You,” which will be celebrating its 25th anniversary in 2019.

There are, of course, plenty of die-hard fans for whom Ms. Carey is not merely an end-of-year highlight but a singer-songwriter for all seasons. This year, Ms. Carey debuted a new Las Vegas revue, and, to celebrate, a group of 36 “lambs,” mostly in their 30s and 40s, boarded a party bus and cruised the Vegas strip for about three hours. On it, they loudly sang selections from Ms. Carey’s catalog. Many were deep cuts. They call themselves “lambs” after the schticky term of endearment Ms. Carey used liberally in the early 2000s, including in voice messages she used to leave for her fans on her website. The flock on this particular evening was racially diverse and split fairly evenly by gender, united in difference by an unyielding interest in Ms. Carey, through all the states of her career.