You should do it any way that works for you.
if you want to enter those "old" diaries into contests, adhere to the rules of the contests - single strain and fill out information each week as best you can split it up per week etc.
sometimes there are rules similar to: "Must be a recent diary" or some sort of time requirement, but they don't often enforce that sort of thing, bwahaha. And, that's all based on when the diary is created. Nobody will know when you grew what you post anyway.