Now That's What I Call Music (also simply titled Now or Now 1) is the first album from the popular Now! series that was released in the United Kingdom on 28 November 1983. Initial pressings were released on vinyl and audio cassette. To celebrate the 25th anniversary of the album and series, the album was re-released on CD for the first time in 2009. However, alternative longer mixes of Only For Love, Double Dutch and Candy Girl were included in place of the original shorter single mixes from 1983. A double vinyl re-release followed for Record Store Day on 18 April 2015. In July 2018, the album was newly remastered and re-released on CD, vinyl and cassette to commemorate the release of the 100th volume of the series.

Based on this passage, when did now thats what i call music come out? Let's give stream of consciousness first:
To answer this question, we should know that: Now That's What I Call Music (also simply titled Now or Now 1) is the first album from the popular Now! series that was released in the United Kingdom on 28 November 1983.
So, the final answer is 28 November 1983.