Ghata (Hari Ghata / Shyam Ghata)
What it is
Ghata means a gathering of rain clouds. On a Ghata day the whole darshan is one colour: Shree Thakorji's vastra and shringar, the pichhwai behind Him and the decoration of the sanctum, so that the darshan looks like massed cloud. Hari Ghata is the green one and Shyam Ghata the dark one. The calendar marks the start of the Ghata days as winter arrives, and Shyam Ghata on the Amaas that follows.
Why we observe it
Shringar in Pushtimarg is never decoration for its own sake: dressing Shrinathji for the season is treated as literal, attentive seva of a living person who feels the cold the way anyone does, adorned accordingly rather than left in clothing suited to a different month. Ghata marking the calendar's turn into the cold season is one more instance of the sampradaya's insistence that seva follow real time and real weather, not an abstract, unchanging ritual calendar.
A Vaishnav does not need to know every detail of the shringar to take the bhav from the day: attentiveness to Prabhu's comfort, expressed through something as concrete as what He is dressed in, is itself a form of love.