As @Gagravarr indicated you've already asked the 2nd question so I won't answer that again.
According to US Customs and Border Protection Website
Your ESTA authorization is generally valid for multiple trips over a period of two years (starting the date that you are approved) or until your passport expires, whichever comes first*. This means that as long as you received an ESTA authorization to travel, you do not have to reapply during the validity period. You may update your travel information; however this information is optional and is not required.
So leaving US for one day to Canada and then coming back is not an issue as long as other terms of the Visa Waiver Program are satisfied.
NOTE Please note that your time in Canada counts against your 90 day allowed in the country under the Visa Waiver Program.