Why do i get depressed one day out of the week and then have a big mood swing that same day?

Because you've convinced yourself that this is going to happen on that day every week.

Hormones... mental fatigue, physical fatigue, stresses both conscious and unconscious, these things can all play with your head. You may be very sensitive to what's going on around you, too.

The important thing is... don't let the swings get to you. They're a part of life. Just find a way to react to them that lets you feel balanced at the end of the week or month. Try to appreciate your moods... you could be worse off (emotionally dead... or dead, period.)

That's what they call................" LIFE!"

You could have some hormonal instability. Have it checked out.

Unless your mood changes are exceptionally fleeting, I believe that it is pretty normal for us to change our moods every so often because situations that we encounter every day affect us in so many different ways. This is what makes us truly human. As long as these moods do not get in the way of your usual routine, I think you just have to cope with it in the best way that you can.

chemical imbalances, hormonal changes, illness, stress, mental disorders, I could go on but I might offend you.

Don't listen to those people telling you are bipolar. Bipolar is serious and it's much more than just a mood swing once a week. You would be having difficulty every day.

What are you eating and drinking habits? At your age, I would say low blood sugar is the likely culprit. Does the depression come on after you've eaten or drunk something sweet? A rush in sugar can cause a dip in blood sugar and make you feel blue or depressed.
Not eating can do the same thing.

I would look at your dietary habits first. Then, look at your activity. Is there something you do on this day that you really don't like to do? Dread can bring on a little anxious feeling.

If it only involves a half day a week, I frankly wouldn't worry about it at all.