What is the chronological order of the episodes of xxxHolic?
1st Season, 24 episodes
2nd Season (Kei), 13 episodes

Movie "A Midsummer Night's Dream", This probably takes place somewhere during the 2nd season (60 minutes, essentially a long episode)

OVA Shunmuki, 2 episodes, also crossover with Tsubasa: RESERVoir CHRoNiCLE
OVA Ro, 2 episodes, occurs a significant amount of time after Shunmuki.