Not a day shall be lost

[Xinfu]

-Not a day shall be lost in learning English.

1. Is it grammatical?
2. Does it mean?:

A. We should grab every minute and second to learn English.

OR

B. We will lose days(=time) in learning English, so we should stop learning it.

[Rusty]

It's grammatical.

This is a tricky question to answer without more context.

'Shall' can be used to express futurity ('will' is the more common usage by today's standards) or it can mean 'must' (obligation). The latter usage is the traditional sense of the word.

If futurity was intended, it means you should learn English, and the day you spent learning it was worth every effort (the day wasn't lost).
If obligation was intended, it means you should not waste a day learning English.

I applied the 'expend your time learning English' sense to your example. However, a grammar source says that 'shall' (futurity) is normally used with the first person and 'will' is used with the other persons.
Observing this 'rule', "Not a day will be lost in learning English" would readily be understood to mean that learning English is a worthwhile expenditure of your time.
The opposite 'rule' applies when obligation is the underlying meaning. "Not a day shall be lost ..." would then be (or should be) interpreted as 'don't waste your time learning English'.

We don't know which meaning was intended by the author of the cited sentence.