Messa Angele
Dear Ceci,
The 42 days program is followed chronologically but you won't start all over if you jum a day. You can use as many days as you want to complete the self'development program. The website keeps tract of your evolution and you continue from there everytime you sign in.
However, meditating on regular basis helps a lot in giving you a better meditation experience and enable you to benefit the most out of it.
Hope this is helpful, PIPO