Maybe we should all adopt Chinese weekday names: Sunday (星期日) remains same, Firstday (星期一) for Monday, Seconday (星期二) for Tuesday, Thirday (星期三) for Wednesday, Fourthday (星期四) for Thursday, Fifthday (星期五) for Friday and Sixthday (星期六) for Saturday. One-letter abbreviations would be simply S, 1 through 6.
Monday - "понедельник", which is coming from the "day after the (previous) week", i.e. after the Sunday
Tuesday - "вторник", true here, has "second" in the name
Wednesday - "среда", has "middle" in the name
Thursday - "четверг", also true, has "fourth" in the name
Friday - "пятница", also true, has "fifth" in the name
Saturday - "суббота", derived from the Hebrew "shabbat"
Sunday - "воскресенье", almost the same word as "воскресение", which is the Christian Church word for the Resurrection (of Jesus Christ)
