If you're in Britain on a Sunday, don't buy a birthday card -- it's against the law

Very soon, if Prime Minister Margaret Thatcher holds firm to her belief in the free-market economy, the English Sunday will be transformed. Her government intends to introduce legislation making Sunday sales legal. It would thus turn back legislative acts designed to protect the sanctity of Sunday which can be traced back at least as early as the Sunday Observance Act of 1781.

If the promised bill makes it through the House of Commons and is not torn to shreds by the House of Lords, some of the most striking shopping anomalies in the world will disappear.

For example, in England a fish-and-chip shop may sell any dish on a Sunday except fish and chips. Other takeout shops, however, may sell as much fish and as many chips as they like. Provided, of course, that they are not a fish-and-chip shop.

Sunday traders stay within the law if they sell postcards. But if a birthday card is sold, they could be fined several hundred pounds.

Lovers of country life may take solace from the fact that under the Shops Act of 1950 they are allowed to buy fodder for horses, mules, ponies, and donkeys, so long as it is supplied by a farm, stable, or inn.

City dwellers delight in the Sunday availability (after 10 a.m.) of shellfish, rolls, and fancy bread, if there's ``a genuine emergency.''

The eccentricity (if not absurdity) of the law has defied a score of attempts to change it over the past 35 years. Religious groups insisting on the sanctity of the English Sunday have been against change, as have the trade unions.

But two years ago Mrs. Thatcher's home secretary, Leon Brittan, appointed a committee to study the matter. It recommended last November that the moment had arrived to abolish all legal restrictions on the hours stores may be open.

Last week Mr. Brittan introduced the necessary legislation with majority support, but parliamentary opponents of Sunday trading promised a rough time when the bill finally reaches the House.

Many of the MPs who oppose the idea of legal Sunday sales are Conservatives. They believe England is already rushing headlong toward a ``seven-day society'' in which Sunday will be no different from any other day.

The implication is that the English day of rest will go out the window along with bizarre Sunday sales regulations.

One opponent declared: ``Sunday is a special day. It offers a pause in the mad dash. Having shops open and selling their wares without . . . hindrance will wipe out 24 hours of well-earned peace.''

Such arguments, Home Office officials point out, tend to ignore reality. Because of the eccentricity of the shopping laws and the difficulty of enforcing them, many shops that should not open on Sundays do so without hesitation.

Huge do-it-yourself stores are among the worst offenders, but they can point to flourishing trade and plenty of shop assistants willing to earn extra money by staffing counters on Sundays.

Opponents also face an awkward argument north of the border with Scotland. There Sunday trading has been legal for some years and, as one of Brittan's officials pointed out, ``The world has not come to an end.''

Even so, the government is having to move with care. Protracted opposition to a Sunday trading bill could waste parliamentary time, and if the Lords force amendments, oddities could yet appear in the final law.

But the ultimate argument of the reformers rests on two basic points: the 1950 Shops Act is so widely breached as to make it a nonsense which gives the law a bad name; and no one would be forced to shop on Sundays.

As for the trade unions, Thatcher and Brittan are set to outflank them by a provision in the law whereby workers could legally not work on Sundays if they so wished.

This provision is likely to be patterned on regulations in force in Massachussets, giving wide protection to workers who want to retain Sunday as a marker in the rhythm of their lives.