Key to a destective story

[Walt Whitman]

Hi everyone.

I’ve been preparing a detective story for my 13-year-old students of a middle school in Italy.
The story is set in a prison where one of the inmates is planning to escape.
He writes a secret message to inform his brother about his escape plan, day and hour. One of the guards finds it out…
I suppose I’m not allowed to post the whole story, so I’d like you to have a look at the message and its key:

on / prison / third / leave / day / I / next / day / week / new / two / before / hours

My students have to decipher it.
The key is: On [the] third day, next week, two hours before [the] new day, I leave [the] prison.
This means on Tuesday at 10:00 p.m.

I chose a present simple with a future meaning (instead of “I am leaving”) because I needed one word only in that space. Do you think it works?

Thanks a lot

WW

[Beeesneees]

The tense isn't the problem. The days are! The third day could be considered to be Tuesday, but is more likely to be considered to be Wednesday. According to some commonly used calendars (esp. in the US), it is the third day of the week. According to international standard ISO 8601, however, it is the second day of the week.

"Two hours before the new day" sounds very odd too.

On Tuesday next week two hours before midnight I leave prison
would be
on/prison/Tuesday/leave/next/I/week/midnight/two/before/hours

and if that is too easy because it mentions the name of the day, you could always refer to the words' origins and use 'Tiw's day' instead of 'Tuesday' and 'middle night' or 'middle of the night' instead of 'midnight'.

By the way 'to find out' is wrong here. It should be
One of the guards finds it.
I mention it in case you plan to use the phrase in your story.
_________________
"Inside every old person is a young person wondering what happened."
Terry Pratchett

[Walt Whitman]

Thank you Beeesneees.

The days are not a problem for my students. I taught them to put Sunday at the start of the week, so “Tuesday” would be the third.
(In Italy we follow the international standard with Monday as the first day of the week.)

New day: “Tuesday” is the day of escape, so the “new day” would logically be “Wednesday” at 10:00 p.m.
I was wondering why you find “new day” so odd.
What would you use? I need two words.

By the way, I thought that “find out” and “discover” were synonyms.

WW

[Beeesneees]

I've given you my suggestions.
'new day' is odd because it's unclear which day you refer to and from what you say about Monday being the first day of the week in Italy, I wonder why you are teaching your students something different.

'find' and 'discover' are often synonyms. Not 'find out', which is a phrasal verb meaning to get some information about something/somebody by asking, reading, etc
The guards find the message.
The guards find out about the message from another prisoner.
_________________
"Inside every old person is a young person wondering what happened."
Terry Pratchett

[Walt Whitman]

I don't know why but in all the textbooks we use in Italy Sunday is always the first day of the week.
That said, do you still think it’s unclear which day I refer to?
Sunday first, Monday second, Tuesday third.

WW

[Beeesneees]

It's unclear for me, but if your students are used to it, I guess it would not be unclear for them.
I would be left wondering whether you were referring to Tuesday or Wednesday, and whether the 'new day' was that day or the day after.
_________________
"Inside every old person is a young person wondering what happened."
Terry Pratchett

[Walt Whitman]

Thank you very much.

WW