請高人幫幫忙，英文電郵!!!!!

Dear xxx,

Thanks for the enquiry from your company regarding the cleaning schedule of the water tank of building last week. We are pleased to inform you that work has completed on (day of week).

Regards,

2012-09-27 12:20:59 補充：
你的簡單回覆也可以，但你原本的寫法會比較完整和更專業地回應客戶之前的問題，你可以自己選擇。

2012-09-28 00:56:26 補充：
'on' a specific date while 'at' specific time.

thanks a lot!

呵即是"The cleaning works has completed (on or at?) last week. "文法也可以, 但我應該用(on or at?) last week, on/at 那個是正確文法呢?