What Nature reserve number was established on 19961220 20.12.1996
CREATE TABLE "table1_26013618_1" (
  "nsg_nr" real,
  "name_of_the_nature_reserve" text,
  "district_town" text,
  "area_ha" text,
  "date_established" text
);
SELECT MIN("nsg_nr") FROM "table1_26013618_1" WHERE "date_established"='19961220 20.12.1996';