I can't stress enough how important it is to do regular vocal exercises.
This is especially true if you are not singing for a few days.
An Olympic runner doesn't wait until he's on the starting blocks to get ready for the race.
We don't see the months of intense training that is carried out before this race.
Like the Olympic runner it is important for singers to learn the correct methods of singing and to exercise the voice in order to keep a good and strain free vocal sound.