this is very unreadable, you know you dont have to use a function library for everything, loops exist and etc.

I know that. In my opinion, when lambda"sum" is used in accumulate, maybe it leads to a problem. Actually code want to assign 0 (the init number) to acc(the integer param in sum), but its function signature is(const char c, int acc), and maybe 0 -> c(the char param), and that's a little bug. You can take a look at the example in the link above. I tried these two verisons, and they all passed the test, so that's my problem.

Have you already realised what's wrong in your question? Those params are refer to lambda "sum" function. Not to accumulate.

I have some trouble to understand your code.

Why not (int acc, const char c) ? IN THE SORT FUNCTION.

The first argument should be "acc" according to
http://www.cplusplus.com/reference/numeric/accumulate/?kw=accumulate

Can you explain to me, thx:)

This problem is very difficult to understand, I still don't understand what the question is.
E.g. ".. after I save 12"... 12 what? days? weeks? units of money? Ah! It's after the saturday where I saved 12 units. Difficult to catch.
Then there is a table of days with missing values... is it zero?
And finally, what's the point of mentioning the planet? Just to number the weeks?

Allows duplicates in result.