Is there a specific norm that most of the software development companies in Germany follow in regards to the date of salary payment for full-time employees? There is no specific day stated in my work contract and I heard some people (not my colleagues) say that it's mostly 'end of the month'. What does this mean? Last working day of the month?
"End of month" means some (typically 1 to 3) working days before the last day of the month. This is to go sure that you have your money at the last day of month* since many regular payments (rent, insurances, loan payments, etc) are due at the first working day of the month.
Some companies also pay mid-month (with the 15th as due date), but with the same scheme, i.e. some days prior to the actual payment day.
*Although handled digitally, bank transfers in Germany still often take one working day, so you want some time buffer.