Date: 202101
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: Marginal
Weather elements / visibility: Icing
Light: Night
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ramp ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion; Fatigue; Situational Awareness
Anomaly: Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown; Ground Incursion Ramp
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: It wasn't supposed to be a very long leg or a long day. The plan was to fly from ZZZ to ZZZ1; grab a quick snack and go to ZZZ2 for the overnight and good food. But; that wasn't what happened.  Snow flurries forced us into unplanned deicing. Bad alternates required reworking and refueling. In the middle of deicing; a sudden snow squall sent visibility plummeting and negating any holdover times. New takeoff alternates then had to be amended onto the paperwork; and re-validating required new paperwork. To describe our departure out of ZZZ as a struggle would be an understatement. But; ever so slowly and methodically; we did eventually depart to ZZZ1. It was on arrival that we were notified that our overnight had changed and to contact scheduling on the ground. So; you can imagine that when we landed in ZZZ1; we were mentally bruised; hungry; tired; and on edge. The landing was normal; as was the taxi to our planned gate. What confused us is what we saw upon turning to face south; looking at [Gate X]. There was nothing. No rampers; or wands or even lights. It was a completely empty and dark. I set the parking brake and asked my FO (First Officer) to send another in range report and commented that we should probably call ZZZ1 OPS and see if they had changed our gate. FO dutifully called OPS on the number two radio so I dialed the north ramp frequency into radio one to monitor. At that exact moment the ACARS message returned as my FO was repeating what OPS had just said; gate change to [Gate Y]! I released the parking brake and began a slow turn to the right when I realized I wasn't at an outstation in the middle of nowhere or on an uncontrolled ramp. I had lost all situational awareness and was in the middle of ZZZ1 moving without permission! I immediately stopped the aircraft and called North Ramp with the gate change. He replied yea there was a gate change and 'did i just do a 180?' I don't recall saying much before he asked 'and are you still moving???' I immediately replied we were not moving. At that point I was told I should never; ever do that and that he had numerous aircraft with beacons on trying to push. I should have apologized right then but instead fumbled though the gate change and having the pushing aircraft in sight. I still can't believe I would make such stupid mistake. I can't believe I didn't apologize. I was just beat up and wanting desperately to just take a five minute brake. Instead I let my guard and awareness down which led me to make a very stupid move.




Synopsis: Air carrier Captain reported taxiing on ramp without clearance and stated they had lost situational awareness as a result of a long and fatiguing day.