I know that problem too well!
In Russian, the numbering is embedded into the names (вто́рник ‘Tuesday’ from второ́й ‘second’, четве́рг ‘Thursday’ from четвёртый ‘forth’, пя́тница from пя́тый ‘fifth’). I’m learning Portuguese which names Monday the second (segunda-feira) and so on, and I keep substracting the numbers of they days when speaking. This is so confusing :x
Sunday (воскресе́нье) is the last day of week in Russia. Here's the calendar for July, 2016 in Russian illustrating the concept:
"Moving to the first floor on the third day of the week." US = Moving to the floor at ground level on Tuesday. UK = Moving to the floor just above ground level on Wednesday. If I've followed things accurately, it seems Russian is a mix of the two -> floors like US; days like UK.
Not sure what "ground level" means, but in Russia, basement is not considered as the first floor. So, there are basement (underground level), first floor, second floor and so on. Sometimes buildings do not have a basement, so there are only first, second, third etc. floors.
Ground level is street level - i.e. the floor one is on when entering the building (unless the building is complicated - cut into a hill, etc.).
This is called the first floor in America and Russia, but not in UK. In UK, one goes up a flight of stairs to reach the first floor.
Not trying to overcomplicate - just have a bit of fun with regional variations.