I wait until mid april when the weather is mild and there arent any more freezes. I dont think you should give them their first fertilizing later than early June because they get a lot of growth started earlier than that so you dont want to miss out on helping that growth along.

The first thing I do is hit them with some Epsom salts and then some K,once the soil temps are well over 50(F) at 4".

As the soil temps approach 60(F),I try to coordinate their first feeding with a nice rainfall and some sunny warm weather.

This rather then going by the time of year-I don't think Palms roots can really absorb the fertilizer until soil temps are above 50(F) anyway.