Short Tuesday-to-Thursday lotteries that reassign cancelled and left-over river launch dates as they come up.
Grand Canyon's own weighted-lottery page says a main lottery each February assigns next year's noncommercial launch dates for 12-to-25-day self-guided river trips, with applications open the first three weeks of February. Follow-up lotteries reassign cancelled or left-over trips, typically opening around noon on a Tuesday and closing at noon MST on a Thursday. We alert when each opens.
Updated , checked by hand against the target's own page.
Watched from nps.gov, the target's own public page — not a reservation system we poll.