Officially, the two Island supermarkets celebrate their centennial on March 10. Both stores are on State Road, one in Vineyard Haven, known as “Down-Island Cronig’s,” and the other, “Up-Island Cronig’s,” is in West Tisbury. There will not be a lot of fanfare on March 10. Cronig’s will celebrate its 100th with a month-long 30 percent Island Card discount to shoppers.