what if there are no ravens but crown of winter and crown of mereen are in play at the same time? 

It is both summer and winter at once. There is no rule conflict from the constant/passive effects from the crowns (with the crowns i cant remember which they are, id go constant but they are probably passive), the only thing that turns them off are a raven of the appropriate type, the ravens song plot card or blanking them (obviously removing them from play as well).

Remember that, in this circumstance, any and all effects that state "if it is winter, do X," or "if it is summer, do X" will apply, while any and all effects that state "if it is not winter/summer…" will be null. Also, any card that states two different effects, both "if it is summer…" and "if it is winter…," you now have a choice of which to trigger… or both will go off if it is passive.

stormwolf27 said:

Oops. Hit publish too soon… if both are going to go off (passive), I believe first player determines what order they resolve in.

All crowns' effects that create seasons are conditional constants (if blah raven is not in play). The Crown of Meereen also has a response and the Crown of Azor Ahai has an any phase, but these do not create seasons.

Khudzlin said:

Careful with your wording. I'm going to assume you mean the triggered response and any phase actions when you say "they" do not create seasons. You're right of course, but, by your wording, it might confuse others who are not as familiar with the rules in this instance.

Well, since I first said the effects that create seasons are conditional constants, I assumed I was clear enough.

I thought all card effects trigger from top to bottom in order? So if says "stand after playing an event if it is summer… kneel after playing an event if it is winter" it would ultimately be knelt.

mdc273 said:

Within a single effect, you go in order, but when you have two different effects, they are completely independent - even if they are on the same card. So in the case you are theorizing, the "if it is summer…" and "if it is winter…" effects are separate and conflicting, so the First Player chooses the order, just like he would if they were printed on different cards.