Do Japanese actually use さくじつ (昨日), こんにち（今日）and みょうにち（明日）instead of their 和語 counterparts? If so under what circumstances do they choose to use which one?
Do Japanese actually use さくじつ (昨日), こんにち（今日）and みょうにち（明日）instead of their 和語 counterparts? If so under what circumstances do they choose to use which one?
Good question though it is a tricky one. Out of the three, こんにち（今日） belongs in a different group than the other two.
さくじつ (昨日) and みょうにち（明日） are most heavily used in business situations, where we tend to speak much more formally than many Japanese-learners would imagine. In any other situations where formal speech is expected, you will use those two.
The "today" version in this group is ほんじつ（本日） and it can never be replaced by こんにち（今日）.
こんにち（今日） means something almost entirely different. Unlike さくじつ, みょうにち and ほんじつ, こんにち has nothing to do with the formality of speech. It is the "today" as in "the coalminers of today", "today's greatest Russian writers", etc. One cannot use こんにち in phrases such as "It is raining today.", "Man, I stole two cars today!", etc.