You can choose to become a stand-by test-taker for the April 2 subject test. Just call the ETS office and express your interest to become a stand-by test-taker.
Otherwise, you have to wait until July 1, 2005 to know the test dates for the 2005-06 test year. Chemistry subject tests are given on 2nd week of November, 2nd week of December, and 1st week of April, all on Saturdays.
Hope this helps.