I made reservations to camp on the 21st, and the confirmation says the weekend. Can we show up on the 20th, and just pay for the extra day?

They ate usually closed tues-thurs to prepare for the next weekend.

Have fun

It's likely ok but it's will be better to go online to change the date while you have access to internet.

You can typically arrive on Friday's with no extra charge. I would call ECETI to make sure this is still their policy.