my first guess is over-watering, try every 3 weeks.

Will do! I'll try every 3 weeks. Thanks!

I'd say your potting mix has probably broken down and hardened, and the water is running through a single channel down the edge of the pot. Potting mix is not permanent, and needs replaced every few years.

Id soak it in the sink or a bucket of water (to the top of the soil) for about an hour, then repot immediately.

btw, that looks more like a Thanksgiving cactus, not a Christmas cactus. I can't see flowers that well, but they are different shape on CC than TC; and CC has more rounded leaves.

It really doesn't matter if a Christmas or a Thanksgiving cactus....for all intents and purposes, they are grown the same.

I agree with mblan's suggestion for repotting. I would also say that watering on any sort of schedule - like every three weeks - makes little sense. Water when the plants need it. And that will be influenced by the potting media, the size of the pot, when it was last repotted and how much heat or sunlight it may receive. These should have plump, turgid leaves or pads. When they start to lose plumpness or become a bit wrinkled or shrunken is when they are wanting more water. Mine, in a similar sized container as the OP's, gets watered maybe once a month in winter.

All very helpful information - thank you to all! I will soak pot and repot as suggested, and then focus on watering only when needed. By gardengal48's description of wrinkled leaves, I'd say mine is likely to be wayyy over watered since I have none of that issue.

I'm assuming for repotting I should use a cactus/succulent mix? Since this is either a thanksgiving or christmas cactus?

Thanks all!!

So what kind of mix are you going to use?

One that flows freely giving the roots plenty of air to breath and one that will allow you to water often, even in winter?

Or one that will clog up and break down rather quickly limiting root function in time becoming compacted and not allowing to water freely and one that holds dissolved salts?

If it were mine, I would break up the soil out of the root ball, rinse off the soil that remains and use a very good porous mix. They by nature grow in trees lots and lots of water.

I let mine dry out between waterings. I don't water it until the pot feels very light in my hand. It has grown slowly but steadily under this regimen. Has never lost any bottom foliage.