Your clients can make a charitable contribution of shares at any time, but it will only be eligible to be deducted in the taxable year in which it is made (which ends each year on December 31). We process donations only on business days when markets are open, so we recommend your clients make a donation no later than the second-to-last business day of the year in which your client wishes to claim the donation. As an example, if your client would like for a donation to be eligible to be deducted on their 2017 tax return (which should be filed by April 17, 2018), make sure to submit the donation on or before Thursday, December 28, 2017.
This information is not meant to be tax advice.