124:16 The practice is not to sit on a chair, neither at night nor during the day until midday. Rather, one sits on the ground and moves to a chair after midday. (One may sit on a cushion rather than directly on the ground; even a low stool would be permitted if sitting on the ground is too difficult – Mishnah Brurah 559:11.) The rest of the prohibitions of Tisha b’Av are in effect the entire day until the stars come out at night.
124:17 The practice is not to start preparing the evening meal until after midday. (One may be lenient in a case of great need – Mishnah Brurah 559:39.) For the sake of a mitzvah, it is permitted.