Grotto Day is celebrated on the feast day of St James of Compostela, 25th July. James, one of the 12 Apostles of Jesus was martyred in AD44 and his remains are thought to be held at Santiago de Compostela. He is known as the patron saint of pilgrims amongst other things including rheumatism sufferers, horsemen and pharmacists. Possibly, as he was said to have been a fisherman, his emblem was a scallop shell and pilgrims wore a scallop shell symbol on their hat or clothes during pilgrimages.
It became a tradition in various parts of Britain to make grottoes and decorate them with shells. The height of their popularity was in the 18th and 19th centuries, with larger shell grottoes being a permanent garden feature. In Margate a grotto of 4.6 million shells still exists in an underground tunnel and a room of 15 by 20 feet (5 x 6 metres). Another shell grotto exists in Torfaen in Wales.
In Whitstable the Oyster festival week in July includes the blessing of the sea on the nearest Thursday to the 25th.
Although decorating a whole dolls house with shells might be a bit much, you could always make a small grotto for a dolls house garden or just place shells around the dolls house on shelves or window ledges to remind the miniature inhabitants of past seaside holidays.