On the 12th day of Christmas . . . Spain celebrates

After Americans have thrown out the Christmas tree and packed away the decorations, Spanish children are still counting the days until Jan. 6 for their presents.

That's the day when the Three Kings of Orient arrive, bringing toys and presents for good boys and girls.

While American shopping centers are ending their after-Christmas sales, the real hard-core commercial pressure is only beginning here. It will climax the week before Jan. 6, when toy stores are open until 10 p.m. and until after midnight on Jan 5.

Although Santa Claus has been gradually creeping into Christmas celebrations, most Spaniards regard old Saint Nick as part of a foreign and vaguely pagan custom promoted by department stores to extend the Christmas buying season. Christmas for Spaniards is a traditional family reunion like the American Thanksgiving.

Even those Spaniards who have adopted Saint Nick are often confused about other countries' Santa lore. They tend to tell bewildered children to listen for ''Papa Noel,'' a fat, white-bearded man dressed in red pajamas who will arrive on a camel loaded with toys. And few here have ever heard of poor Rudolph, the red-nosed reindeer.

Christmas trees have been more easily assimilated as part of Spain's Europeanization. They are prevalent in stores and are becoming more common in homes as a seasonal decoration.

But the greatest excitement now centers on miniature nativity scenes set up in every home. Every day, children move the figures of the Three Kings closer and closer to the manger. On Jan. 6, they will wake up and run to the nativity where the Three Kings have finally arrived and have left presents next to an old shoe of each child.

Throughout the first week of January, overcrowded department stores will offer not Santa Claus but the Three Kings in person to hear requests from endless lines of awed children. The grand finale is, of course, Three Kings Eve when large cities stage major production parades of the Three Kings that could rival any American Santa Claus parade.

But much more charming are the living nativities organized in every small village the night of Jan 5. For instance, preparations are already well underway in Villanueva de la Canada, population 2,500, just 30 miles from Madrid. Like other towns throughout rural Spain, the nativity is reenacted every year.

Local children dressed in traditional Castillian shepherd costumes and carrying two foot long candles begin the parade. Live sheep, compliments of one of the genuine bonafide local sheperds, are herded behind the children. A series of angels follow and then a teenage Virgin Mary seated precariously on a real donkey is led by a bearded Joseph. Mary has her hands full clutching a plastic baby doll while trying to hang on. Tractor floats carrying Roman soldiers, more angels and shepherds follow, flanked by excited children.

Finally, trumpets announce the long awaited, grandiose arrival of the Three Kings, mounted majestically on horseback and dressed in a dazzling mixture of royal medieval and Arabian attire. Equally royal looking footmen of the Three Kings ward off hoards of children and remind them that they can hand in their letters at the manger. An out-of-place Santa Claus throwing candy from a tractor , helped by the local beauty queen and accompanied by a transistor radio blaring popular Andalusian Flamenco Christmas carols, ends the parade.

Local dignitories lead the whole village to the makeshift manger set up under the porticoed porch of the town hall where Mary and Joseph are now seated next to the donkey and corraled sheep. The Three Kings take their seats while children line up with their letters.

Each child gets a chance to state requests and to solemnly promise good behavior. The three lines move quickly while the footmen and a priest maintain order - and Santa Claus showers the crowd with candy and balloons from the balcony of the town hall.

Loudspeakers blare out more Flamenco carols and eventually the Three Kings announce they are about ready to go all night delivering toys to good children. Bad children receive chunks of coal.

With this last admonition the children are reluctantly dragged home and coaxed into bed, after carefully placing their shoes under the miniature nativity and leaving something to eat for the camels of the Three Kings who had been resting during the parade.