my fever has been getting worse these past few days.从句子中可以看到

如果具有时间意义的名词如：morning,day,afternoon,night,week,month,year,century等之前有any,every,next,first,last,this,that,following等词加以修饰时,在句中用作时间状语时,可以省去相应的介词.
My fever has been getting worse （during） these past few days.