"Planning for next year" vs. "planning for the next year"

Next in this usage is better regarded as a determiner rather than as an adjective. ( http://simple.wiktionary.org/wiki/next ) (compare last year, this year). The use of another definite determiner (the) is thus superfluous. However, this does not apply with following, present, and previous, where the definite article is needed to pre-modify:

I wanted to start planning for the following year.

I'd guess that following etc are behaving more like adjectives, so there needs to be a definite determiner (the definite article) added. Next grades into adjectivalness, so the definite article is an option. With this, the adjectival component does not exist, so we can't say for the this year.

In your particular example, I would say that next year refers to 2013, while the next year refers to the twelve-month period ending at the end of November 2013. (Not much difference in practice, but more if you use the phrase in February). I can't explain the difference, but I'm sure it's there; perhaps it's just idiomatic.