My 1998 blazer doesnt want to start up

I have a 1998 Blazer 2wd,4dr 3 months ago in the winter I started to have problems starting it up in the mornings but in the day it would crank
right up. Now it is summer and I havent had any trouble starting it up in the mornings or during the day. on Saturday it started again it takes about 3 to 4 seconds to start but on the road it doesn't jerk back or anything it runs great. Can anyone help me what is wrong with it?
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