'Tis the season for buying calendars, of course, so this might be the right time to get yourself one of the most useful "accessory" programs for personal computers: a built-in, instantly available appointment calendar.

I'm a big fan of programs that put ordinary desktop tools -- notepad, calculator, Rolodex, dictionary, etc. -- inside the computer. One of the essential daily tools that adapts most happily to computerization is the appointment calendar.

Frankly, if you only use your PC for occasional brief tasks, you're probably better off relying on a plain old low-tech paper calendar or one of those thin pocket-size appointment books.

But anyone who spends more than half an hour a day at the computer keyboard will find greatly enhanced by an electronic appointment calendar that can pop up instantly whenever you need it, or warn you of crucial meetings or deadlines.

There's a plethora of appointment calendar programs on the market for IBM computers and their clones. Many of them are part of larger "desk accessory" programs. (In the Macintosh world, this tool had not caught on as well because the Mac doesn't accommodate pop-up utilities quite as easily as MS-DOS and OS/2 computers.)

The granddaddy of this genre is Borland's Sidekick program, which I have used with satisfaction for years. Sidekick has a calendar that jumps right on top of whatever tasks you're working on with the touch of a key combination. It initially shows the calendar for this month, with today's date highlighted. As soon as you hit the return key, an hour-by-hour appointment list for today pops up next to the calendar. Using the arrow keys, you can move quickly to days, months or years in the past or future.

Sidekick's calendar is fast, reliable, and a cinch to use -- but it's not perfect. I would like to see a monthly calendar that highlights every day on which there's an appointment, and I'd like an alarm function. Some of these shortcomings will be fixed when the new Sidekick Plus comes out; meanwhile, though, you can find good alternatives.

There's an appointment calendar in the Lotus Metro utility program, the excellent Sidekick clone program from the makers of 1-2-3. It has a lot of features, but it's not as easy to use as the Sidekick calendar.

Metro's appointment book first pops up showing today's schedule; to get to a monthly or weekly calendar, you have to go to a menu and enter a series of keystrokes.

Unlike Sidekick, Metro has an alarm; unlike Sidekick, it has an easy mechanism for keeping track of recurring appointments, like the budget meeting that convenes every fourth Monday. I like Metro, but it is neither as fast nor as intuitive as the Sidekick calendar.

For sheer speed in popping up on the screen or moving about to different dates, it's hard to beat the Calendar .exe acessory that comes with Microsoft Windows.l

The built-in calendar in Windows also pops up first as a daily appointment schedule, but you can quickly shift to a monthly calendar. The monthly view will highlight any day that has an important appointment. Thus you can tell in a single glance -- without leafing through each day's schedule -- which days next week you'll be free to skip out of the office and go skiing.

Any of theses three programs will provide a nice electronic appointment book for your computer, but each of them is a full package. You can't get the Microsoft Calendar .exe without buying the whole program -- Windows -- to which it's attached; the same goes for Sidekick and Metro.

I've found a few low-cost, stand-alone calendar programs. At the extremely low end of the price spectrum (i.e, free), are two programs you can download from various bulletin board systems: Cal.arc and Remind.arc. Both of them do the job, but just barely.

And then, for $20, there's a well-thoughtout pop-up appointment calendar that I can almost recommend: Byte Size Calendar, from Software Resources (Sunnyvale, Calif., 408-738-4311).

This program jumps up instantly when you hit the "ALT-C" key combination. It shows a full calendar for this month, with today's date highlighted and today's schedule off to the side.

All dates with important appointments are hightlighted, and those schedules automatically come onto the screen as you move the cursor through the calendar.

In another corner-of the screen there's a running "To-Do" list with reminders of important tasks that you have to finish; when you've completed one of these "To-Do" chores, you strike it from the list and it is saved automatically in a disk file of your completed tasks.

Byte Size Calendar is a little frustrating to use because it is not ye "well behaved;" that is, it doesn't fit in well with other memory-resident programs, and thus tends to freeze your computer.

It's also hard to load the program automatically when you first turn on the computer. But when these glitches are remedied, Byte Size Calendar will be a terrific little PC tool.