Shouldn't it be "We are open (everyday) except on Mondays." ?
We ask businesses "Are you open/closed?", not "Do you open/closed?".
Not all around the world. "Do you open on Monday" is perfectly acceptable to me, as is "We open on Monday" is response do GERG below.
why is “di hari Senin” used instead of “pada hari Senin” ?
We ARE open except on Mondays