Four weeks at a time, ranked by when the tide drops far enough, for long enough, in daylight. No ads, no accounts.
The tide is the whole story here, so unlike a fishing calendar this site needs a NOAA station and says nothing useful without one. A day is rated on its best stretch of low water: how far the tide drops, how long it stays down, and whether that happens in daylight.
How low counts most. Exposing the low intertidal is the trip, and the difference between a -1.4 ft low and a +0.8 ft low is the difference between anemones and a wet walk. Because a good low means different numbers on different coasts, the threshold is set from the station's own predicted lows rather than a fixed height: -1.0 ft is a red-letter day in San Diego and an ordinary Tuesday in Anchorage.
Daylight is close to a gate, not a mild preference. A -1.8 ft low at 4 AM is a superb tide and a useless outing for most people, and it's rated accordingly. Night tidepooling by torchlight is real and rather good, so such days keep a fraction of their score rather than dropping to zero.
Duration matters on its own. A flat, slow low gives you two hours on the rocks; a sharp one gives you forty minutes. Two days with the same low-water height can be genuinely different trips, so the window drawn on the chart is the whole stretch below the threshold, not a fixed hour either side of the low.
Moon phase is shown but deliberately not scored. New and full moons do produce the biggest tides, but NOAA's predicted heights already contain that, along with the moon's distance. Adding a phase bonus on top would be counting the same thing twice.
Plenty of days score zero, and that's the honest answer. Roughly a fifth do. On the Pacific coast the extreme lows swap between daytime in winter and the middle of the night in summer, so half the year is simply better than the other half, and a calendar that pretended otherwise would be lying to you.
Get there early and watch the water. Walk out on the tail of the ebb and follow it down rather than arriving at the low itself. Surf and swell matter enormously for both safety and visibility and aren't yet modelled here, so check a marine forecast before you go, and never turn your back on the sea.