Slight ammendment to deal with week 1.

=IF(IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))="Mon",TEXT(J4,"dd/mm/yyyy")&" - "&TEXT(J4+14,"dd/mm/yyyy"),IF(IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))="Tues",TEXT(J4-1,"dd/mm/yyyy")&" - "&TEXT(J4+14,"dd/mm/yyyy"),IF(IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))="Wed",TEXT(J4-2,"dd/mm/yyyy")&" - "&TEXT(J4+14,"dd/mm/yyyy"),IF(IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))="Thurs",TEXT(J4-3,"dd/mm/yyyy")&" - "&TEXT(J4+14,"dd/mm/yyyy"),IF(IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))="Fri",TEXT(J4-4,"dd/mm/yyyy")&" - "&TEXT(J4+14,"dd/mm/yyyy"),IF(IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))="Sat",TEXT(J4-5,"dd/mm/yyyy")&" - "&TEXT(J4+14,"dd/mm/yyyy"),IF(IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))="Sun",TEXT(J4-6,"dd/mm/yyyy")&" - "&TEXT(J4+14,"dd/mm/yyyy"),"")))))))



Also realised you may be looking at 26 fortnights as opposed to 52-53.... I found a solution, but it doesn't handle years with 53 weeks.. (cannot seem to upload the file)

Column Z =WEEKNUM(J4)

Column AA =ISODD(Z4)

Column AB =IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))

Column AC =IF(OR(WEEKNUM(J4-0)-WEEKNUM(J4-1)<-1,WEEKNUM(J4-0)-WEEKNUM(J4-1)=1),"Sun",IF(OR(WEEKNUM(J4-1)-WEEKNUM(J4-2)=1,WEEKNUM(J4-1)-WEEKNUM(J4-2)<-1),"Mon",IF(OR(WEEKNUM(J4-2)-WEEKNUM(J4-3)=1,WEEKNUM(J4-2)-WEEKNUM(J4-3)<-1),"Tues",IF(OR(WEEKNUM(J4-3)-WEEKNUM(J4-4)=1,WEEKNUM(J4-3)-WEEKNUM(J4-4)<-1),"Wed",IF(OR(WEEKNUM(J4-4)-WEEKNUM(J4-5)=1,WEEKNUM(J4-4)-WEEKNUM(J4-5)<-1),"Thurs",IF(OR(WEEKNUM(J4-5)-WEEKNUM(J4-6)=1,WEEKNUM(J4-5)-WEEKNUM(J4-6)<-1),"Fri",IF(OR(WEEKNUM(J4-6)-WEEKNUM(J4-7)=1,WEEKNUM(J4-6)-WEEKNUM(J4-7)<-11),"Sat","")))))))