When is the first glucose test normally done?

Celeste

This is my second baby. I think my first was done at 28 weeks but I don’t remember. My doctor gave me a lab order for the test at 24 weeks but failed to tell me when I am supposed to go in and do it. It doesn’t say on the sheet. It’ll just be the normal 1 hour test I believe. I will definitely call him but wanted to get an idea first.