From The Motley Fool:
Real estate: If you purchase real property under an unconditional
contract, you begin counting on the day you received title to the
property or the day after you took possession and assumed the
incidents of ownership, whichever is earlier. Taking delivery or
possession of real property under an option to purchase, however, is
not enough to start the holding period. The holding period cannot
start until there is an actual contract of sale. Likewise, the
seller's holding period cannot end before that time.
I understand this as stating the execution of the contract both starts the holding period for the buyer and ends it for the seller. The phrasing seems vague with the "cannot" part, but I read this as this step is when it happens and there is no way to make it earlier.
If you want a truly thrilling read, you can read the 26 U.S. Code 1223 - Holding period of property which may provide additional information or links to other US Codes that specify further.
From the IRS website (emphasis mine):
Short-Term or Long-Term
To correctly arrive at your net capital gain
or loss, capital gains and losses are classified as long-term or
short-term. Generally, if you hold the asset for more than one year
before you dispose of it, your capital gain or loss is long-term. If
you hold it one year or less, your capital gain or loss is short-term.
For exceptions to this rule, such as property acquired by gift,
property acquired from a decedent, or patent property, refer to
Publication 544, Sales and Other Dispositions of Assets; or for
commodity futures, see Publication 550, Investment Income and
Expenses. To determine how long you held the asset, you generally
count from the day after the day you acquired the asset up to and
including the day you disposed of the asset.
Disposing of an asset in real estate typically refers to date of contract execution, reinforcing my thoughts from the first article.