I have been noticing a lot of ghost types spawning everywhere. Is this because of it being Halloween time? Does the time of year largely affect what kind of Pokemon show up most frequently in areas for a period of time? Will I still be able to find fire types in the snow or will it be ice types only?
Yes. There is a Halloween event right now that causes ghosts and dark types to spawn more in the wild.
To be honest, unless there is an event, the spawn rate doesn’t seem to be based on seasons. It is, however, based on weather. If you click the weather icon, it will tell you what types would normally spawn more often.Sunny weather, for example, spawns more fire types, snowy weather spawns more ice types, etc.
The current event does alter that a little, due to the extra ghosts and dark types.
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Oh! Forgot to cover holidays! There is usually an event in effect during most major holidays. In December, for example, it will rain iced types regardless of weather everywhere as part of the Christmas event. It will also be the only time you can catch Delibird and it’s shiny variant. 
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