For some people it can start any where from 1-2 weeks before their period.... when mine first started up I use to become moody, emotionally sensitive, depressed and the worst of them suicidal the week prior to my period and like you said the day after I started my period all was good again.

Just look at like this..... were I only got one week out of every month where I ate 2 pieces of dark chocolate - you get two weeks of eating it.... basically you eat the chocolate when you are suffering from symptoms of PMS or PMDD.