In Germany, there is a statutory minimum holiday time of 4 weeks per year: 20 working days for a 5-day week, 24 working days for a 6-day week. Your employment contract or collective agreement states how much holiday you get. It is not possible to agree on less holiday than the statutory minimum amount.
When you have been employed for 6 months you have full holiday entitlement (from the entire working year). Before that, you are entitled to part of the holiday: this is calculated according to the number of days of holiday per year and the length of your employment (in months).
You must take your holiday in the year you worked in. If you were unable to take your holiday in one year due to urgent business or personal reasons, you can still take it until 31 March of the following year. At the end of the year, your employer must tell you how many days of holiday you have left and request that you take them.
If he/she does not do this, the days of holiday will still remain and you will have to take them the following year.