One language per person.
I speak four languages fluently and can switch between them in a blink of an eye. I learned them all in my early childhood. I believe that knowing more languages has no negative effects except if the social circle deems it somehow elitist. My wife also speaks several languages. Even so, we have limited our use of language towards our 2-year-old son to one language per person.
My wife speaks only German with him, and I speak only Danish with him. He hears me speak both Danish and German with other people. Because we live in Austria, there are exceptions when I speak German instead of Danish, but only when others need to understand what I'm telling my son.
I could speak three or four languages with my son, but I choose only one in the early years so that he can learn that one well (he will learn another from my wife). I think I would confuse him if I constantly switched language without context or reason.
I'm no expert on this but I would hesitate to use any particular weekday as context for an "English day." My reasoning is that small kids have no concept of "weekdays" so they don't see any context even when you know that the context is "weekend".
If you really want to do this, then perhaps you could connect it to something that small kids understand, like "English after sundown" or "English at the playground."
This bilingual upbringing is easy for us because the country we live in doesn't involve a third language (e.g. if we lived in England). But I believe that good English skills are immensely important in our international society, so I will add English to the mix in a year or two, or definitely when school starts, but I'm not sure yet how to approach this (stay tuned on this site!).
Also, all our computers have only English software because I prefer it that way. This is not an "educational decision" but will support the learning of English when the time comes.