I water my potted palms every single day and give the palms in the ground a soaking pretty often as well. The Sabal minor maybe gets water every 3-5 days or so, but my trachy is in a very warm location so it gets water every 2-3 days. They should always be moist until established, after that they can go every now and again with less water. No real answer to how many times to water because when it's 95F outside, you have to water every day, but if it's in the 70s, you really dont have to water much at all unless it's very dry.
-Alex

On warm(83 and up) dry days I water every evening when I get home. When the sun comes out and the humidity is under 30% they dry out in know time. if it;s really humid they will not dry out quickly (if at all if it rains on and off). My cutt off at watering every days is if it's dropping below 60 at night.

Everyday and they still seem dry!

According to most studies plants begin to be stressed at 86 degrees farenheit so i try to water if predicted highs are above that. Around here that's nearly all summer long. Certain palms need far less though. Desert palms like washies i almost never water except to fertilize. Check out the ahs heat zone map for your heat zone.

I think there is some misunderstanding regarding palms native to desert or Mediterranean climates. In particular, palms such as Washingtonias ans Phoenix dates depend on subterranean sources of water fed by springs . Their deep roots tap into aquifers. Obviously, in a container, they are completely restricted and rely heavily on our relatively shallow waterings. I would not suggest keeping these palms dry in the growing season, especially if you are feeding. My Washingtonias and Dates are grown in 18 gallon tubs in full sun, outside, in a hot microclimate that averages 90 -95 f. this time of year. I hose them down every day and by the following day, they need more water. In less sun, cool temps, larger tubs, more rain, etc., they would need less watering. Atmospheric water (humidity), is a different issue for these desert/Med. palms. Some are very intolerant of the humid East/Southeast!--Especially, W. filifera and many of the date palms. Attached is a link regarding the very particular homeland of filifera and their reliance on aquifers, springs, oases, and riparian environments.

Here is a link that might be useful: Washingtonia filifera

Darn....I thought that link was to your Washys(-:

My palms including washies are all in the ground. If they were in pots then yes i would definitely water often as i do all potted plants.