CREATE TABLE table_16963 ("Series no." FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production Code" FLOAT)

When did the series number 23 air?
SELECT "Original air date" FROM table_16963 WHERE "Series no." = '23'
SELECT "Original air date" FROM "table_16963" WHERE "Series no." = '23'