1 (usually with the) coming straight after sb/sth in time, order or space: The next train to Baltimore is at ten. The next six months will be the hardest. the next chapter Who’s next? the woman in the next room I fainted and the next thing I knew I was in the hospital. (informal) Round here, you leave school at sixteen and next thing you know, you’re married with three kids.
2 (used without the) ~ Monday, week, summer, year, etc. the Monday, week, etc. immediately following: Next Thursday is 12 April. Next time I’ll bring a book.
(Advanced Oxford dictionary)

But I can't find any differences between these two definitions.What is the difference in meaning?
- Next six months will be the hardest.(=?)The next six months will be the hardest.
- Next Thursday is 12 April.(=?) The next Thursday is 12 April.


I think the difference in meaning that they're highlighting relates to the words "immediately following", which means "immediately following in time, starting now". Here are a few points relating to this usage that immediately come to mind. I don't claim that this is in any way complete.

In "next + noun" combinations, "the" is normally omitted only in the "immediately following in time" sense, and only with named days and months, seasons, and a few specific words such as "week", "month", "quarter", "year", "time", etc.:

Next Monday is my birthday.
I'll be 40 next July.
Next summer we're going on holiday to Japan.
It's going to be hot next month.
Next year will be tough.
Next time I see you, I'll be married.

You cannot use "the" in this sense with named days and months. For example, "The next Thursday is 12 April" is wrong. However, you may be able to do so with words such as "week", "month", "year", "time", etc.:

The next year will be tough.
The next time I see you, I'll be married.

But it depends. You would not, for example, usually say "It's going to be tough the next year."

When referring to events in the past you can use "the", even with named days and months. Randomly Googled examples:

The following winter the galleys lay on the Loire, but the next summer they cruised on the east coast of Scotland...
The next Monday we went to the soap market together...

For expressions consisting of a number and a time period, you have to use "the". For example,

The next six months will be the hardest.
Our biggest challenge is the next three years.

"The" may also be omitted in certain special idioms (such as "next thing you know"). However, with most other "next + noun" combinations, you usually have to use "the" (or any of various words that can take the place of the article), though there are almost certainly other exceptions that I haven't mentioned.

The next train leaves in an hour.
He's the next Marlon Brando.
Where's our next meal coming from?
JesusenglandWhat is the difference in meaning?
None. There is no difference in meaning. The difference is usage.
General principle:
Use next when your reference point is the present moment. Otherwise use the next. For example:

Next Thursday means 'counting from today'. The next Thursday means 'counting from some other time -- not from today'.
Next Thursday is August 14. I had lunch with Dianna on August 1. The next Thursday was August 7.
Students: We have free audio pronunciation exercises.
I can't believe it is so simple. CJ, you did it again!

Just to make sure I get it right:

Let's get on next train. (Now, I feel 'the' is neccessary Emotion: sad )

We went to the station and got on to the next train.

Did I get it right?
Oops! I oversimplified. My remarks pertained to the use of next and the next when referring to time. Trains are different! Emotion: sad
Both of your examples require the next (train). trainis a countable, singular, concrete noun, so it has to have a determiner. This takes precedence over any other considerations. Emotion: smile
Time expressions are adverbial, so We're leaving next Monday is OK. Monday is noun-like, but it's used as an adverb. And besides, it's not a concrete noun anyway.
Thanks, CJ.
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How about "We'll remember this choice the next time you post."?
What does " the next time ." in the sentence mean?