Would it be possible to change the date of payment from Septemebr 19th to September 20th because the 19th is a national holiday?
Is there anyway we could postpone our payment until the 20th of this month because the 19th is a public holiday?
あまり仰々しくない言い方で、でも支払いの延期をお願いするなら、次のように言うのが良いと思います。
ーWould it be possible to change the date of payment from September 19th to September 20th because the 19th is a national holiday?
「19日は祝日なので、支払日を9/19から9/20に変更してもらうことは可能ですか？」これはある程度丁寧にお願いする言い方です。
ーIs there any way we could postpone our payment until the 20th of this month because the 19th is a public holiday?
「19日が祝日なので、支払いを20日まで延期することはできますか？」
こちらの方がもう少しカジュアルな言い方です。
ご参考まで！
カナダ 
