This is a preventable feeling, as the only panic you should really feel when homeschooling is when you're ordering books for school and you realize you have too many choices.  Here are a list of vital ways to keep your sanity and make the first day a good one (and the first week fun):

1. Pick a definite day to start.  Don't just say, "Ok we'll start sometime when we get back from vacation." Most likely the public school schedule would work best, but it doesn't really matter as long as you feel confident your are getting the required number of weeks for your state.

2. Set up a place to study that is best for the kids, and not what looks nice to you.  We will be doing school on a couch, we keep the homeschool books in a plastic crate, and our other books are nearby.  We have a craft area next to us as well, and toys are out of site but close enough for the preschooler to get to without feeling alone.

3.  Keep it simple.  Unless you are radically unschooling, you should plan what books you want to use and a general idea of how much you want to do in a day.  I find though that trying to get down to the details of planning what pages you are reading and what projects you are doing on paper for a whole year just isn't realistic and sets you up for failure.  If you don't finish a book in a year, it's no big deal.

4.  Get dressed the first day of school, and have the house tidy.  It's possible that as the year goes on you will probably be doing days in your pajamas, but make the first day special by having everyone get dressed and make all the beds and do the dishes before school.  It makes the kids treat the day with more respect and clears your own mind.

5.  If it's the first day of homeschooling ever, work gradually into a full schedule.  Only do math and reading the first day, then add handwriting, then science a few days later.  Or do a bare-bones schedule for a week, then add everything else the next week so you can work into a routine.  If you made a schedule for the day of which order to do a subject, and someone gets frustrated, scrap the schedule and come back to that subject later.

6.  Start at the same time every day.  Some homeschooling families are so busy they find this impossible and do school in the car sometimes.  But really if you get your brain used to studying at the same time each day it makes it easier.

7.  Make sure it's not all about the curriculum.  Sometimes I have seen families so into their expensive curriculum that they have no money left for the stuff that their kids want to do in their spare time.  For example, if a child really wants a microscope, make sure you haven't used up your budget!

8.  Be cheap.  Buy used, online and at library book sales.  Only buy the books that you can't get at the library unless you use them all the time.  Use the internet!  Use the computer!

9.  Be creative.  Your child hates math but loves computer games?  Get them into programming.  Even a young child can do basic programming.  Having an awful day?  Go on a spontaneous nature walk field trip.

10.  Have fun, enjoy your children.  You get to be home and learn with them.  It is a gift.  Even when they are driving you insane, you will laugh later.  You will.