Many foreigners with Korean bank accounts find themselves financially strapped and, in some cases, stranded on the two biggest holiday weekends of the year: Chuseok and Lunar New Year.
On holiday weekends, the safest way to travel around Korea is with cash or your non-Korean credit card. Banks shut down access to funds--in other words, completely shut down--at odd times during the holidays. Even if an ATM is open, you won't be able to access funds. And if you're in a bar, cafe, convenience store, airport, hostel, et al., during these closures, your check card will not work. You might find yourself stranded and cashless for several hours. Or, like many foreigners last year whose bank closed for the entire holiday weekend, you may be broke for four days and looking for help to eat.
To give you two examples:
Kookmin Bank (KB) will be down from 10am-1045am on Saturday, 2/13, and from 1201am-8pm Monday, 2/15;
Woori Bank will be down from 4am-7am on Saturday, 2/13, and on 11pm, Monday 2/15, to 2am, Tuesday 2/16.
Most banks' Internet Banking is likely to be down until Tuesday morning. You should make sure you know when your check cards won't work. As you can see from above, it's different for each bank. Woori Bank customers are not likely to be inconvenienced; on the other hand, Kookmin customers have an entire 24 hour period without bank access.
Korea4expats is a decent website for getting general information about daily life in Korea, but it (like most sites about living in Korea) offers no information about holiday closures. Hagwons and Public Schools do not inform their employees. Your Korean friends are not likely to know. Go into your bank and insist they give you the times. Or, if you can read hangukmal, visit the Korean language site for your bank and get the information. I can tell you that for Woori Bank, its English language site does not contain the closure info.
Please be sure to check with your banks before leaving tonight or tomorrow morning for your holiday excursions. I suggest calling them. The web sites can be confusing and just because you don't find information about closures, doesn't mean they aren't scheduled.