Answer by taking a quote from the following article:

Mazzy Star is an American alternative rock band formed in Santa Monica, California, in 1989 from remnants of the group Opal. Founding member David Roback's friend Hope Sandoval became the group's vocalist when Kendra Smith left Opal. Mazzy Star is best known for the song "Fade into You" which brought the band some success in the mid-1990s and was the group's biggest mainstream hit, earning extensive exposure on MTV, VH1, and radio airplay. Roback and Sandoval are the creative center of the band, with Sandoval as lyricist and Roback as composer of the majority of the band's material.

The first substantive confirmation that the duo would reconvene to complete work on their fourth studio album came from Sandoval herself in a July 2009 interview with Rolling Stone magazine, where she was quoted as saying, "It's true we're still together. We're almost finished [with the record]. But I have no idea what that means." Later, in a September 2009 interview with Vancouver-based music website Straight.com, interviewer John Lucas wrote of the 8-year gap between Bavarian Fruit Bread and Through the Devil Softly, "That seems like a long time until you consider that Mazzy Star, Sandoval's partnership with guitarist David Roback, hasn't put an album out since 1996. Sandoval promises that will change; she and Roback have their very own Chinese Democracy in the works, but it won't see the light of day until The Warm Inventions have wrapped up their tour." On October 12, 2011, Hope Sandoval's official website confirmed the duo would release their first new material in fifteen years later that same month. The double a-sided single "Common Burn"/"Lay Myself Down" was released digitally on October 31, 2011. A limited edition blue-coloured 7" vinyl was also announced for release on November 8, though a manufacturing delay resulted in its release being pushed back to January 24, 2012. Their fourth studio album was expected to be released in the latter half of 2012, following completion of a tour earlier in the year. The band completed an 18-date Californian and European tour in 2012, their first since 2000. Performing at several major European festivals, the band consisted of original members Suki Ewers and Keith Mitchell, and were also joined by Sandoval's Warm Inventions band-mate Colm O Ciosoig and Keith Mitchell's son Paul, whose band the Brook Lee Catastrophe also served as the opening act of select shows. Pedal steel guitar was performed by Josh Yenne. After the final date of the tour in August 2012, David Roback stated that production on the album had completed and that it would see release "soon". In late 2012, several unreleased song titles composed by Hope Sandoval and David Roback were registered with the band's long-time publisher BMI, including "Flying Low" and "Spoon", both of which were performed multiple times on the tour. The band also launched an official merchandise store. On July 13, 2013, the band announced details of their fourth studio album, Seasons of Your Day, which was released on September 23, 2013 in the UK, followed a day later on September 24 in the US. The album reached a career-high No. 24 on the UK albums chart. The band began a North American tour on November 3, 2013 in support of the album. On April 19, the band released two new songs as part of Record Store Day 2014. "I'm Less Here" and "Things" were released on 7" vinyl, with the run limited to 3,000 copies worldwide. On December 22, 2014, a 40-second clip of a previously unreleased song was posted on to the band's official Facebook account.

what is the reformation?
On October 12, 2011, Hope Sandoval's official website confirmed the duo would release their first new material in fifteen years later that same month.