Limited to the summer season

July 1st 2015
Today was rainy. ("Today was a rainy" is technically an incomplete sentence. Rainy is a word that describes a subject, and "a" is used to lead into another subject. If you end with "a rainy", you end up ending your sentence before it actually intends to end. Without the "a", the subject is still "Today", and because of this: "rainy" describes "Today".)

I bought some beer that is on sale for a limited time in the summer. In Japan, there are a lot of limited-time products.
I think it's uncommon in the world because we have four seasons. 
How lucky we are! We can enjoy four variations of products :)   (You kind of caught yourself here, though you need the "<em>s</em>" at the end because you are talking about multiple product<em>s.</em>)

Limited to the summer season

July 1st 2015
Today was a rainy.

I bought some beer that goes on sale for a limited time in the summer. In Japan, there are a lot of limited-time products.
I think it's uncommon in the world, because we have (only?) four seasons.
How lucky we are! We can enjoy four validation of products :)

(the meaning of validation here is unclear)