History of Last Thursday

Last Thursday is a Legendary Strand Pulse Rifle (energy slot) from The Edge of Fate, a 390 RPM Adaptive Frame. TheGamer suggests Demolitionist with Frenzy for PvE and Moving Target with Elemental Honing for PvP; its high horizontal recoil makes Arrowhead Brake close to essential.

Acquisition

Dropped by enemies and chests on Kepler (in-game source: "Exploring Kepler"), or targeted through Focused Decoding at the Altar of Relativity using Fated Ciphers and Phaseglass Refractions (Game Rant). Like all Kepler weapons it carries the "Exhaustive Research" origin trait, which loads micro-missiles when aiming at enemies. Sources do not mention a craftable pattern or Adept version.