During normal days, I sleep at 12AM and wake up at 6AM, but during holidays, I go to sleep at 2AM and wake up at 9AM. How can it affect my health??

Not too bad... One should try to go to bed at 11:00 pm if waking up at 6:00 am, to get 6.5-7 hours of sleep. One the weekends, catch up by getting up 2 hours later. This sleep pattern is not bad (not harmful) if one is happy, smiling, enjoying life, and not drowsy in the daytime.