오늘은 휴일이 니가요 => 오늘은 휴일이니까요. * -니까 (and -니) means because or since. 바람이 부니(까) 나뭇잎이 떨어진다 (The wind blows causing leaves to fall). 오늘은 휴일이 합니까요 => This is completely wrong. You can't connect 휴일이 and 합니다/합니까 directly. Maybe what's meant is 오늘은 휴일이기도 하니까요 (As today is also a holiday). * XX+이기도 하다 = XX+이다 (is XX) + -기도 하다 (it is also ...) => (It) is XX as well. - 서울은 수도이고 한국 최대 도시이기도 해요 = Seoul is the capital as well as the largest city in Korea.