Urgent question about Covid-Tests to travel to Germany

I just found out that you now have to have already a negative Covid-Test result before traveling into Germany. So my friend and I are traveling on Sunday. How in the world can we provide this negative result? I mean, if the test is only Friday afternoon, do I have a guarantee that I will have the result on Saturday? I intend to do it with Synlab.

Is there an alternative with antigen tests? If so, where?