When's the best day to write?
To post?
When's the best time of the day to send an email?
When's the best time to call?
When's the best time?
The best time is the time where you don't have to think and decide about what's the best time anymore.
That's why (if you can) the best time is daily.
But of course you can't do daily.
Which is exactly why you have to do it daily...