If your cycle is irregular, you don't need to wait for a specific day - book your appointment whenever suits you.
Your hormonal results are always interpreted within your overall context, and our medical team takes cycle irregularity into account when reviewing them.
If you can, note roughly where you are in your cycle (or that it's irregular) so the interpretation is as accurate as possible.
If your cycle is regular, see When should I do my blood test if I have a regular cycle? for the ideal timing.