I also would have initially agreed with Monday morning start of business, but keep in mind that your email will be lost amongst many others and more likely to be 'cleaned' away... so I suggest 10-11am where the recipient has, by then, in theory cleared out the clutter.
In this age of information access nearly ubiquitous and pervasive across so many devices and locations, I think it's much less of an issue though... depends who your target reader is (exec, family man or woman, and cultural differences)...
Some cultures would absolutely never (or almost) check business emails at night or weekends, in others it's almost a given.
