You should have a notification of the start of the seasonal festival on the 2nd day of said season, at about 7am or so. Also, you shouldn't need to manually change the lots out, the festival lot should auto-place.
Edit: Also, just because I'm bossy
please keep in mind that thread titles should be indicative of what you are posting about - in this case, something like "question about festivals" or some such.
If you want help with something make your topic title as descriptive as possible.
And finally, welcome to the forums!