When is it obligatory to pay Zakat on money saved to buy a house: Is it when the Hawl (lunar year) is completed during the same period, or on Ashura, and should it be paid from the money deposited in the bank or from the salary, or must the money be withdrawn from the bank to pay Zakat?

Zakat is obligatory on saved money if it reaches the a nisab (threshold) and a hawl (full lunar year) has passed on it according to the Hijri calendar. If you pay Zakat on all saved money for buying a house when its hawl is due, that is better. It is permissible to pay the obligatory Zakat from any money you own. It is not permissible to deposit money in an interest-based bank except in cases of extreme necessity, such as the absence of an Islamic bank and the fear of losing the money.