Mr Lang said his grandfather's brothers also served in World War I.
He said he supported the right to debate the flag, but not for the alternative flags to fly on a public building on what's been called New Zealand's darkest day.
"She's come out and she's said she wants the flag changed. That's her right but [don't] do it on Passchendaele day, our worst disaster ever," Mr Lang said.
"She should get a slap on the knuckles."
A council spokesperson said the day was chosen in consultation with various agencies, and depended on the availability of staff to hoist the flags, as the flag poles were on a part of the building that required safety equipment.
"There was no disrespect intended. Our overriding intention was to contribute to an important discussion for New Zealand."
In a statement yesterday, Ms Wade-Brown said she had initiated the move because it allowed the public to view the flags "in action".
"Flying the five alternatives as well as the current New Zealand flag provides an opportunity for Wellingtonians to compare each when they are fluttering in a brisk capital breeze," she said.
"We'll be able to choose which of the designs looks best in action, instead of just in print, online or on screen. This will help the public make an informed decision about the choices in November."
RSA National President B.J. Clark said he understood Mr Lang's concerns and said flying the flags on the day was just "very unfortunate".
"People need to be very careful with when they do these things, because they will upset passionate people if it's done at the wrong time," Mr Clark said.
"Passchendaele is one of these very significant days in our military history. We lost far too many people in one day."
In Wellington there was little to mark the 98th anniversary of the battle, but Auckland remembered with a ceremony yesterday at Auckland War Memorial Museum.
