WJSN was introduced with a Zodiac concept, meaning hypothetically they would have 12 members representing each of the 12 Zodiac signs. (There's some stretches and fans later dug up some falsified birthdays, but all of that is explained in the member directory.) They were introduced in 4 units: Wonder, Joy, Sweet, and Natural.
WJSN officially debuted with 12 members on February 25th, 2016 with MoMoMo, but their first stage and group introduction was held at KT GiGa Legends Match 5 days earlier (watch here)! Their music show debut stage was MCountdown on February 25th, where they performed an intro version of Catch Me and MoMoMo.They later released a music video for Catch Me.
Some notable events from this era are.... a glitch in the MoMoMo MV, their low first-day album sales, that incident of fansites fighting in front of the girls, slipping on stage due to the rain, and just generally promoting for way too long.... but! They also got some lovely photoshoots, did a collab stage with EXID, performed an iconic version of MoMoMo with a dance break, and showed off a bit with Say Something a number of times. They also started their first reality show! Variety from this era can be found on the video directory.
In July 2016, Starship hinted at a 13th member with a constellation teaser of Ophiuchus, and later confirmed that they would be adding Yoo Yeonjung to the group. In August, they made their first comeback (including a MelOn Premiere Showcase) as a 13-member group: Secret! (Only after a controversy about the MV filming location, of course.) They also released a self-cam MV for Secret, a special R&B version of the song and a self-cam MV for The Secret b-side Robot.
This era was relatively smooth, but some notable events are Cheng Xiao's gold medal at ISAC, a whole bunch of photoshoots, and a lovely Immortal Songs performance! Technically included in this era is their Girl Group Medley Stage from Music Bank's year end special and their Gayo Daejun stages: Secret (KBS), Ultra Dance Festival (SBS). They also did a few more collabs, including Cheng Xiao in Sunny Girls, Yeonjung with Yoo Seungwoo and Exy with High 4:20. As for MoMoMo, variety from this era can be found on the video directory.
At the very end of 2016, WJSN released teasers for their 3rd comeback. They hosted a comeback countdown prior to the release of the MV for I Wish. They later released a ballad version of the song. (Also, though Mei Qi is in the MV, she was absent for most of promotions due to movie filmings in China.) Music show promotions lasted a whole 9 weeks.
Some notable events from this era are winning the EPK Discovery of the Year Award at the Seoul Music Awards (which also included this performance of Secret and I Wish) and the announcement of their fanclub name: Ujungs! They also had a nice Dispatch HD shoot and got really into Pokemon GO. Then there was... a twitter joke gone viral about Mei Qi and Xuan Yi being lesbian billionaires (with a followup)... I won't give any opinions on it, but that's where that reference comes from. As for Secret, variety from this era can be found on the video directory - though it's worth noting that Yeonjung was active with I.O.I at this time, and those videos are not covered in this primer.
In April 2017, WJSN announced their first solo concert, Happy Moment. (Cameras weren't allowed, but there are lots of fancams regardless! There were some cool unit stages so I'd recommend checking them out.) They later announced that their first full length album would also be titled Happy Moment, and had a full-length comeback showcase prior to the MV release for the title track Happy. WJSN also released a Housewarming Version of Happy with Ip-Deok Live and a CF song for Kiss Me Cosmetics.
Some notable events from this era are their first time at KCON LA (regular stage + cover stage with sf9) and the announcement of their official colors! They also did a collab stage with Kim Jongmin of Koyote and a dance change with Chungha! Not all is well: they had trouble with fansigns and there was some controversy among fans when someone pointed out that Happy had some similarities to a music video by Japanese girl group Perfume (no statement from Starship on this). At least they have their first official fanmeet! As for I Wish, variety from this era can be found on the video directory.