As far as I am aware if Ramadam finishes on 20th September then the banks and government offices will be closed for 3 days 21st, 22nd 23rd Sept. Last year at one stage they did actually close for the full week but i can't remember which one it was.
Seker Bayrami is roughly the equivalent of Easter or Xmas as far as the closing of banks and government offices are concerned. It's no big deal if you're prepared for it.
This year Seker Bayrami falls on 20-21-22 September. Many banks and offices are likely to close early on the 19th and take a half-day holiday
Other holidays coming up soon are :
28-29 October (28th is a half-day) Republic Day.
27-30 November Kurban Bayrami / Feast of the Sacrifice