5 or 6 daily bar in a week

Hi :

I am new to Calgo, so please forgive me if i ask this stupid question, but can anybody tell me why there are sometimes 6 bar and sometimes 5 bars in a week (if you take a look at the daily bar chart). I've tried a little exploration, it seems sometimes it breaks sunday into two bar (16:00 - 17:00 as one bar, 17:00 to next day 17:00 EST as another bar), not really sure if i see the right thing, can anybody kindly give me some hints?

Thanks