How often we are forced to abandon the old customs. The pressures of urban living make it difficult to visit a meadow on Midsummer Eve, there to dig up a piece of coal from under the root of a mugwort.

But how else can we guarantee that we will spend the year free from plague, carbuncle, lightning or ague?

We can adapt, that's how, and instead of giving up the ways of our ancestors, we can learn how to modify them. On June 23, a week from Thursday, Midsummer Eve will be upon us, and with a twist here or there, we can celebrate as others did in earlier times.

Give a late supper and a bonfire dance in your backyard. The custom of burning midnight bonfires on Midsummer Eve, dancing around them and occasionally, in a fit of good spirits, leaping over them was once almost universal.

"Those sacred fires kindled about midnight, on the very moment of the solstice . . . (were) a religious ceremony of the most remote antiquity, which was observed for the prosperity of states and people, and to dispel every kind of evil . . ." wrote Court de Gebelin, an 18th-century student of mythology.

The fires originally were made of bones, according to the Oxford English Dictionary, which gives bonefire as the origin of the word bonfire.

''Wyse clerks knoweth well that dragons hate nothyng more than the stenche of brennynge bones . . ." explained one anonymous chronicler.

Dragons may no longer be a rampant evil, but you could ask your butcher or the family dog to contribute a bone to make the bonfire authentic.

Lay out a cold buffet, plenty of cheap champagne (unless you are rich, in which case, why stint?) and move your stereo speakers out into the back yard. There your guests can leap over the fire or dance to soft music. Not all neighbors may want to be awakened at midnight to celebrate the summer solstice.

If you have a Neighborhood Watch group, sponsor a Neighborhood Watch parade, with the local crime fighters gathering in force to strut through the neighborhood. You will be following another Midsummer Eve custom--a Marching Watch, when the night watchmen paraded through town bearing torches, joined by the citizens and nobles they had spent the year protecting. And if a few burglars happen by and see how very many people have opted for law and order, perhaps they will decide that if crime is to pay, it must be carried on elsewhere.

Have a midnight wine and cheese party, and make sure that at least one of your guests is an unmarried woman. Back in the days before single's bars and computer dating, women had other means of identifying a future husband. One was to fast all day on Midsummer Eve, and then at midnight, to set out a meal of bread and cheese. The front door was left open and soon a man would enter, pick up a glass of wine and toast the waiting hostess. Then, bowing, he would set down the glass and leave--to reappear later in the year to marry her.

Unless you are having the midnight wine and cheese party in the same neighborhood where there is a Marching Watch, it might be better to keep the door closed. We are, after all, adapting these customs to modern times, and it would be a poor-spirited spirit who couldn't make it through a locked door.

Have a cycling picnic. Rent bikes for all your guests, prepare backpack picnic lunches and choose a nearby site to cycle to. If it's hilly, all the better, for you are following the custom where people rolled a wheel about to signify that the sun, occupying the highest place in the zodiac, was beginning to descend. Often the wheel was taken to the top of a mountain and rolled down. As it rolled toward the bottom of the hill, bad luck rolled away with it.

Explain to your guests as they huff and puff toward the crest of the hill, that in a minute or two, as they coast on down, they will be assuring themselves a lucky year.