As I bleed nearly whole month sometime the bleeding stops, but it stops only for one or two days so i observe 7 days of a month as hayz. As now my date which I observe will start from the night of 23 may, so can I take it from the night of 24 so that I observe fast on 24, generally the bleeding at the start is very low?

If the bleeding has continuity (even if blood is present in the vagina) such that you are not clean for even 10 mins and 30 days have passed since the last hayz you cannot change the date