CREATE TABLE table_61173 ("Round" TEXT, "Main date" TEXT, "Number of fixtures" FLOAT, "Clubs" TEXT, "New entries this round" TEXT, "Prize money" TEXT)

What was the main date of the round with 20 fixtures?
SELECT "Main date" FROM table_61173 WHERE "Number of fixtures" = '20'
SELECT "Main date" FROM "table_61173" WHERE "Number of fixtures" = '20'