It is a shame that conditional formatting is so antiquated. To use the condition within a formula one only needs a single dynamic array formula = (start<=day)*(day<=end)
entered in cell C2. Because conditional formatting does not accept named ranges as the region to which the formatting is applied, much less dynamic ranges, nor does it apply Boolean array calculations correctly, one is reduced to relative referencing. I defined a named formula 'hightlight?' by = (@start<=@day)*(@day<=@end)
and based the conditional formatting on that.