A B757 FO RPTED THAT HE AND THE CAPT HELD AT THE WRONG FIX IN PHL'S AIRSPACE.

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ENRTE TO PHL; OUR FLT WAS CLRED TO HOLD AT CEDAR LAKE (VCN) BUT WE ENTERED HOLDING AT SEA ISLE (SEI) INSTEAD. CONTRIBUTING FACTORS: THE BIGGEST FACTOR WAS THAT THIS FLT WAS THE SECOND WK IN A ROW OF FLYING THE BACK SIDE OF THE CLOCK. THE FIRST WK WAS SPENT FLYING LFT-JAN-SDF-BFI ONE NIGHT AND BFI-SDF-JAN-LFT THE NEXT NIGHT. THE HOLDING ERROR ENRTE TO PHL OCCURRED ON THE SECOND NIGHT OF THE SECOND WK. I HAVE TO FLY AGAIN TONIGHT AND JUST LIKE LAST NIGHT (THE NIGHT OF THE ERROR) MY BODY FEELS OUT OF SORTS. ENRTE TO PHL I HEARD HOLDING WAS BEING GIVEN TO THE ACFT IN FRONT OF US; SO I ENTERED THE HOLD INTO THE FMS. THE HOLDING FIX I USED WAS SIE INSTEAD OF VCN. BEFORE I COULD DOUBLECHK WHAT I ENTERED; ATC GAVE US A DSCNT AND XING RESTR. BECAUSE I NEEDED TO GET INTO THE FMC TO ENTER THE XING RESTR; I EXECUTED THE HOLDING; THEN ENTERED THE XING RESTR. THE XING RESTR GIVEN BY ATC WAS NOT A GOOD ONE. WE STARTED DOWN IMMEDIATELY BUT STILL HAD TO PUT FULL SPD BRAKES OUT AND INCREASE THE SPD; OUR DSCNT RATE WAS 6000 FPM OR GREATER. TOWARDS THE END OF THE DSCNT THE AUTOPLT STARTED TURNING THE AIRPLANE R FOR NO REASON. THE AUTOPLT WAS DISCONNECTED TO KEEP THE PLANE FROM TURNING. AFTER THE PLANE WAS BACK TO A MORE NORMAL FLT; THE AUTOPLT WAS RE-ENGAGED. THIS IS THE POINT THAT WE GOT THE HOLDING INSTRUCTIONS AND USED THE HOLDING ALREADY PROGRAMMED INTO THE FMS. BECAUSE IT WAS ALREADY PROGRAMMED IN THE FMS; AND THE EARLIER DISTR BY THE DEMANDING XING RESTR; I DID NOT DOUBLECHK THE HOLDING. WHEN WE ENTERED HOLDING AT SIE WE RPTED TO ATC ENTERING HOLDING. EVEN THOUGH WE WERE NOT NEAR VCN (THE ASSIGNED HOLDING FIX); ATC DID NOT CATCH THE ERROR. ON THE TURN OUTBOUND; ATC QUESTIONED OUR TURN AND WE REALIZED OUR MISTAKE.