It would be the dry season which is generally April to October so that you can get the best out of your time without having rain dampening your activities. But it is still possible to travel in the rainy season as long as you already prepare for it.
As for events and festivals, you may want to look out for major celebrations like Ramadhan and Christmas which are celebrated nationwide, Weisak (Buddha‘s birthday) with procession around Borobudur and Nyepi (Hindu New Year) with people flocking Bali. You may well choose to avoid these dates, check on lunar and yearly calendar to confirm the dates, if you want to avoid the crowds.
Office hours start from 8 AM to 4 PM, or 9 AM to 5 PM. Lunch break occurs between 12 noon to 1 PM. Usually offices are closed on Saturdays, including government offices. Government office hours start at 8 AM and end at 4 PM. This fact may affect the traffic within the Jakarta.
Standard banking hours are from 8 AM to 3 PM from Monday to Friday. However several banks open their branches in hotels (and some in malls) longer than office hour, a few are open on Saturdays so you might want to check first. Jakarta has a number of international banks, even though you can also exchange currencies in some hotel cashiers and official money changers.
The Indonesia Rupiah is also called IDR. Information of daily exchange rate can be found in newspapers or from the net. Some Indonesia banks provide this on their websites. IDR and US$ are the most acceptable currencies. Most tourism resorts have money changer facilities. When you are traveling to remote areas it is advisable to exchange your money and clear your check. Credit cards are only acceptable in big hotels, restaurants, supermarkt, shops and traveling agencies.
Electric power supply is 220 volts in all regions. So be careful with your 110-volt electronic equipment. The sockets will only fit with with two pins rounded-tip plugs (technically known as Type C, E, and F) or use adaptors. Most hotels and many restaurants in large cities provide internet connections or free WiFi.
Since the weather is quite warm in Indonesia, the light clothes would be suitable. It is ad viced not to use very open clothes for two reasons. One, the skin need to be protected from the direct sunlight, since in Indonesia the sunlight is quite abundant. Second, the community will respect you if you dressed properly according to their standard.