If you thought figuring out time zones was bad, wait until you run into the conflicting calendars of expatriate work. July 4th is coming up, a big holiday for the United States where they set off fireworks, grill meat, and celebrate their independence from well, someone. Luckily this falls on a Saturday, so you can schedule that conference call July 3rd, right? Careful there - July 3rd is the observed national holiday, which means just about everyone in the US takes that Friday off, and many will even leave early on Thursday the 2nd to make it a longer weekend. On Monday, they're catching up from the work they missed, and the same is true for Wednesday the 1st.
July 1st has its own set of problems, as that's Canada Day, in Canada, which is not an independence holiday, but it is a national holiday, which means remote offices from Canada, the US, and the UK are going to have trouble connecting that week.
Those are the scheduling conflicts you get when you have three countries with common ancestry. I don't know the holidays in the Czech Republic, but I'm sure they're different than those in Hyderabad, as are customs in which holidays you work and which ones are play. We do know, for example, that International Talk Like A Pirate Day still involves some level of work.
What to do? First - just ask. Local employees know what holidays exist, and they also know which ones are national laws. They'll also be helpful knowing which ones include shutting down of the postal service and closed banks. Second, TimeAndDate.com has a list of all the holidays by country, which means you can do a little research and not have an employee assure that National Rutabaga day means no one is taking any calls. (#protip - the internet search for Rutabaga Day and Rutabaga Month aren't official sources).