How do I determine the last day to terminate under my client’s 10-day option period in the One to Four Family Residential Contract (Resale)?

Since Paragraph 23, the Termination Option Paragraph, uses the word within when describing the time period, Day One of the option period is the day after the effective date of the contract.

For example, if your client’s effective date is January 22 with a 10-day option period, the option period will end on February 1. If your client wishes to terminate under the Termination Option Paragraph, she must provide notice to the seller by 5 p.m. local time where the property is located on February 1.

My client wants to submit a back-up offer on a property that’s already under contract. I see that Paragraph B of the Addendum for Back-Up Contract (TAR 1909) is for the contingency date when the first contract has to terminate or else the back-up contract terminates. What date should I put here?

It depends on how long the back-up buyer wants to stay in the back-up position. Some back-up buyers may want to have their contract terminate within days if the first contract doesn't terminate early, while others may want to retain their back-up contract rights until after the last possible date that the first contract might close. If your client wants his back-up contract to last until or beyond the first contract's closing date, you can also ask the listing agent to provide the first contract's closing date. 

When calculating deadlines for my buyer in the One to Four Family Residential Contract (Resale), should I include weekend days or federal holidays?

Yes. TREC and TAR contracts use calendar days, not business days. This includes weekends and holidays.