The argument here is tenuous to say the least.
In secondary Option X7: delay damages at clause X7.1 it uses the words "for each day" and not the words "for each Working Day". Therefore there's no argument to be constructed for calculating damages based on a Working Day as those words are not used in X7. Presumably they also don't appear in Contract Data part one instead of the standard wording that says the "amount per day"? Unless use of the term Working Day elsewhere in the contract is specifically in the context of calculating delay damages I don't think it is relevant. A day, is a day, is a day i.e. including weekends, bank holidays and Christmas / Easter shut downs - it means literally every day.
In secondary Option X7: delay damages at clause X7.1 it uses the words "for each day" and not the words "for each Working Day". Therefore there's no argument to be constructed for calculating damages based on a Working Day as those words are not used in X7. Presumably they also don't appear in Contract Data part one instead of the standard wording that says the "amount per day"? Unless use of the term Working Day elsewhere in the contract is specifically in the context of calculating delay damages I don't think it is relevant. A day, is a day, is a day i.e. including weekends, bank holidays and Christmas / Easter shut downs - it means literally every day.