I would think that a "worse month" would depend on what you are allergic too. Allergy to grass pollen would make you worse in the fall, ragweed I believe is in the summer months and dust mite allergies are all year round.
I got told from my allergist that dust mites peak in june and it feels the worse come winter (october-november) when you start spending more time inside with the dust mite dander.
Each person is different on the length and severity, I personally react very badly to humdity and here in Alberta, the worse months for that is july-august when we get more thunderstorms and it knocks me to exhaustion from having trouble breathing.
Good thing you don't live here in Vancouver ;-)
For me I start to feel miserable come October (dust mites/molds and cooler air I'm sure). I have to increase my puffers at that point and remain through out winter till just upon summer again and then I can reduce. I'm just about all year round except maybe 3 - 4 months. And the winter cold air does a number on me too, especially if the asthma is not well controlled. My daughter on the other hand has a lot of pollen allergies along with dust mites and molds, so winter and particularly spring is just bad for her.
I'm not sure what time is bad for me, since I was diagnosed in may. I know now that summer is not good for me (I'm not allergic to any grass or stuff, I'm allergic to the smell of them). I've been out of breath for about 2 weeks (heat wave). I'm not good at all with humidity, exercise, perfums, molds, and illness (that's all I know for now), so I'm pretty scared about winter (I'm always sick in the winter). I guess I'll just have to see.
I know that my dad is bad in the summer (I don't know his allergies, I only know he has some).