When an item is taxable, it is best practice to set it up as taxable in the Catalog. Go to Store & Forms / Catalog.
All changes to SKUs that are currently in a form MUST be made at the form level. If you change the tax status at the Catalog, it will not affect the SKUs that are included in a Form. To update the tax status and other sales information for each SKU, go to the Sales Information tab for each Line Item on the form.
Items Not Included in a Form
These SKUs should be updated at the Catalog.
Any items in the shopping cart will be updated to reflect the new tax status.
Setting Your Tax Rate
Make sure your tax rate is correct! Go to Store & Forms / Configuration / Sales.
For temporary "Tax Free" events, you can turn off sales tax for all of the items on a form. From your Form, go to the Advanced tab. Don't forget to reset it to "no" once your event is over. Remember, permanent changes should always be made in the Catalog.