Hello,
My native language is French so don't worry about your English,
Sorry when I tried out the formatting my cell didn't have a border so it was working perfectly.
I don't think you can set the conditional thickness of the border unless using vba code.
The border line only red on top is not possible because you can only change color as a whole [4 sides] and can't change each border individually.
However try this conditional formatting and see if it could do the trick:
We suppose the date are in column F still
It will be a double formatting,
Condition1)
If a Sunday and cell underneath Monday
=AND(WEEKDAY($F2,2)=7,WEEKDAY($F3,2)=1)
red border but only bottom border
Condition2)
If a Monday and previous cell Sunday
=AND(WEEKDAY($F2,2)=1,WEEKDAY($F1,2)=7)
black border but only bottom,right and left ,
black font and bold to attrack attention to the Monday Cell
That will give you the red at the top of the first Monday
The only border missing will be on the Sunday right and left border but if you can live with it That is a solution
Have a try and see if this works better for you