December 16 or later:
DECEMBER 16, 2009 : (MUTALLAB TRAVELS TO ETHIOPIA AND THEN GHANA WHERE THE RETURN TICKET FOR HIS TRIP TO DETROIT IS PURCHASED- BUT THE RETURN DESTINATION IS CHANGED FROM ACCRA IN GHANA TO LAGOS IN NIGERIA) The return ticket for his trip to Detroit, via Amsterdam, was bought at KLM’s office in Accra on December 16 for $2,831 (£1,770). Unusually for a potential suicide bomber, he took the trouble to change his return destination, replacing Ghana with Lagos. He arrived in Nigeria on Christmas Eve. .------------ "Abdulmutallab's bomb plans began with classroom defence of 9/11," Times Online ^ | 12/27/2009 | Dominic Kennedy
...so he purchased it after the strike on Yemen.