can you say 'I'll contact you once again come near March' or is it 'when March comes?'? especially in a formal email to someone important.
"Come near March" is ambiguous, do you mean a short time before March, as in towards the end of February? Then say that instead.
toward(s) the end of February
British English speakers tend to favour towards while Americans prefer toward.
If instead, you mean as soon as March comes, then say
I'll re-contact you come March.
Re-contact removes the necessity of saying "again", and in my view, sounds more formal.
Interestingly, Oxford Dictionaries say the preposition come is informal. I'm not sure I would wholly agree with that but I see no good reason why the OP should not use this word.
When a specified time is reached or event happens.
- ‘And, likewise, a Republican defeat now would only make them leaner and stronger come 2008.’
- ‘We can only hope for a repeat performance of last week come this weekend.’