Tax Code § 34.01(r-1): an in-person tax sale takes place between 10 a.m. and 4 p.m. on the first Tuesday of the month, or the first Wednesday when that Tuesday is January 1 or July 4. Where the commissioners court has moved sales online, bidding may start earlier but must close at 4 p.m. on the same first Tuesday (§ 34.01(a-1), (r-2)).
To get a deed you either register as a bidder with the county assessor-collector, where the county has adopted § 34.011 (Harris and Cameron, for example), or show an unexpired written statement from the assessor-collector that you owe no delinquent taxes in the county (§ 34.015; up to $10, valid 90 days). Ask for it early: Tarrant says about two weeks, Denton five business days.
After the sale: the owner can redeem for 180 days, or two years for a residence homestead, agricultural land or a mineral interest, paying the bid, recording fee, taxes and allowable costs plus 25% (50% in a two-year redemption's second year) (§ 34.21). Money above the judgment goes to the clerk of the court, and claims must be filed before the second anniversary of the sale (§§ 34.03, 34.04).