Since your good day seems to be Saturday, it probably is a result of conditioning during childhood. All week long, kids look forward to Saturday, which means they don't have to go to school and can play and have fun. Obviously, if you look foreward to something, and it goes well, inevitably there will be a let-down afterwards. You don't have anything to look forward to, so you fall into the depression hole. Eventually, as the week progresses, you start looking forward to the weekend again, and your mood rises. Then it's Saturday and the cycle starts all over again.
Count yourself lucky that you have friends to do things with, events to look forward to. I moved back to my hometown five years ago, and I only had two friends left in town. After a few months, they both wigged out and stopped talking to me completely. Seems they had busy lives and couldn't be bothered with any of my problems. The only thing I really have to look forward to is a once-a-year convention of Dolls collectors that I attend every year. It is a big expense, but it's the only thing I have to look forward to. I don't think I'll get to go this year, because my parents would have to pay for it, and I seriously doubt that's going to happen.
I did finally join a local club of Jimmy Buffett fans, but most of the members are slightly older than me, and make way more money than my disability pays. They have lots of social events, but they usually always cost money that I just don't have, so I can't attend events like I would like to do.
Basically, that pretty much leaves me with nothing to look forward to indefinitely. I would stay that I just stay in the depression hole. Unfortunately, I have a bad back and can't sleep a lot of the day away. I've been going to a therapist for years, the last one for about three years. Now I attend a group therapy session that lasts ten weeks. Every week we have to take a depression level test. I've had the same score for three years, the whole time I've seen the therapist. Well, let me take that back, I did come up one point over three years.
Count your blessings that you have friends and events to attend. I have chronic pain, which severely limits my ability to do things, especially things that require leaving the house and walking around for any amount of time. I'm pretty much homebound, and on the rare event that I do attend, I pay dearly for it the next day, painwise.
I definitely think you need some anti-depressant medication, and possibly something for anxiety and/or panic disorder. These kinds of medications are good for leveling out your highs and lows. I would try these out first, as it doesn't really sound severe enough to be bi-polar.
Good luck and let us know how you are doing.