Swear no oaths to others or to yourself, on New Year's Day or on any other day

When I have most need to employ a friend, 

Deep, hollow, treacherous, and full of guile, 

Be he to me! This do I beg of heav'n, 

When I am cold in zeal to you or yours.

-- Buckingham in William Shakespeare's Richard III (as known to Samuel Johnson)