WLIT-FM 93.9 began playing only Christmas music at 3 p.m. Tuesday. The station is in its 25th year of hosting the Christmas music format. The first song of the season had a special Chicago connection — it was “Christmas Vacation” by Mavis Staples (which featured prominently in the start of the 1989 film “National Lampoon’s Christmas Vacation” ).

Why? Listeners love it.

For the Chicago-based adult contemporary radio station, the weeks leading up to Christmas are, in terms of ratings, the most wonderful time of the year.

Lance Venta, publisher of RadioInsight.com , says the fall ratings period helps stations like WLIT determine when to bring holiday music to the airwaves.

“The reason stations switch in early November is so they can get a ratings boost for the final few weeks of the s

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