CREATE TABLE table_25068 ("District" TEXT, "Vacator" TEXT, "Reason for change" TEXT, "Successor" TEXT, "Date successor seated" TEXT)

The successor for the Massachusetts 20th district was seated on what date?
SELECT "Date successor seated" FROM table_25068 WHERE "District" = 'Massachusetts 20th'
SELECT "Date successor seated" FROM "table_25068" WHERE "District" = 'Massachusetts 20th'