I THINK I NOTICED it the first night. It was the night we went to school to meet the principal and the faculty and the wonderful woman who teaches my son. It was the night we bought the school calendar and hung it over the phone in the kitchen and flipped the page to October and it was then I noticed it - 'Prof. Dev. Day.' It's been driving me crazy ever since.

It's your basic calender - the month divided into little boxes for the days. I hung it up over the one my wife gets every year from her old college, the one that features sayings of faculty members - "Truth is an overturned lamp" - and pictures of the campus covered either with leaves or snow - "Old Haverhill under a blanket of white." That calendar used to drive me nuts. Now it's the school calendar. It's all because of Prof. Dev. Day. - No School.

Tomorrow is Prof. Dev. Day. It is the sort of day we all had when we were in school - days when there was no school and the teachers did their thing, got developed. Wonderful Day. Great Day. At the time, I was for more and more teacher development days, maybe weeks and weeks of them. Then absolutely perfect teachers would teach you for maybe three weeks a year.

Now, every time I go to the phone I wind up staring at that school calendar, carrying on a conversation with someone, but finding myself zeroing in on Prof. Dev. Day. It really started to bug me. I mean, what is it that they do? More importantly, why do they have to do it on Oct. 30 - on a school day? What will happen to parents who work? Who will watch the kids? Will some parents have to stay home from work? Will some of them have to pay for child care for the day? Will some kids fail to bring their notes home and be sent to school that day? I can tell you the answer to that one. It's yes.

Prof. Dev. Day became an obsession with me. Does the post office have to close down a day for professional development? Imagine that - no mail for a day while everyone is instructed in the use of the new, improved zip codes. Okay, though teaching's not the same as delivering mail, but teaching is the only thing I can think of that calls out for a day while everyone gets professionally developed. Banks don't do it and insurance companies don't do it and even the District government doesn't do it, and God knows it could use some professional development.

So I called the school board and asked someone there what Professional Development Day was and I was told that it is a day when teachers and principals and others are professionally developed. They go to seminars and meet in groups and, in many wonderful ways, get to be better teachers. It sounded reasonable enough, then I asked why this day could not be held in July or August or even, perish the thought, during one day of the Easter vacation.I did not have the temerity to suggest that Professional Development Day could be held on a Saturday or, say, after school even though I did an awful lot of developing after school when I was a kid. No one could answer my question except to say that it was in the contract.

Now I had to admit there was an edge to my vice. Washington, you see, has the shortest school year of any jurisdiction in the area - 186 days. Its teachers work the shortest day - 6 1/2 hours compared to 7 1/2 elsewhere in the area - and they get, in case you were worried, the longest lunch hour and the highest starting salary. In fact, a Washington teacher with nothing more than a B. A. and no credits toward any advanced degree can expect to make $20,852. You could not, in all fairness, expect someone like this to become professionally developed on a Saturday.

Now, for all this, Washington does not have a school system to write home abot. The test scores are an embarrassment, a tragedy - what used to be called a human tragedy before the term became a cliche. It would be wrong to blame all this on the teachers. They are only teachers - not parents, not social workers. There is not an awful lot you can do with some kids who live in poverty, are raised terribly and learn all the wrong things all too early. Teachers are not miracle workers.

But they are teachers, and the facts are that in Washington they do less of that than they do in most other places. It should be the other way around. It's for this reason that Prof. Dev. Day becomes important. It's a symbol of what's wrong with the school system; a symbol of how the system looks after the system in many cases and forgets about the kids.

So tomorrow is the long-awaited Prof. Dev. Day. It won't bother me any. I've made my plans. But in talking to someone at the school board, I was told how every time they have this day, kids show up for school anyway. You can sort of picture them, little kids who didn't know there was no school, looking around in panic, having no where to go - no parent probably at home. Who ever is in the office can take them over to the bench that's always there, kneel down, and explain.

Tell them it's Prof. Dev. Day.