Monday 25th May now confirmed as the first day, with the 08:50 from Stirling and 16:36 from Euston. The timetable then builds over the following weeks as below (pasted here for the record, from
https://www.lumo.co.uk/plan-your-journey/our-timetable).
25th May - 18th June
Weekdays: southbound 08:50 departure / northbound 16:36 departure
Saturdays: southbound 08:49 departure/ northbound 16:19 departure
Sundays: southbound 13:20 departure/ northbound 19:35 departure
19th June - 4th July
Weekdays: southbound 08:50 & 17:09 departure / northbound 10:16 & 16:36 departure
Saturdays: southbound 08:49 & 17:51 departure/ northbound 10:16 & 16:19 departure
Sundays: southbound from stirling: 13:20 departure
southbound from preston: 10:40 departure
northbound: 14:18 & 19:35 departure
5th July - 26th July
Weekdays: southbound from stirling: 08:50 12:22 & 17:09 departure
southbound from preston: 06:41 departure
northbound: 06:11, 10:19, 16:36 & 19:19 departure
Saturdays: southbound stirling: 08:49, 13:21 & 17:51 departure
southbound preston: 06:04 departure
northbound: 06:10, 10:19 & 16:19 departure
Sundays: southbound from stirling: 13:20 & 16:34 departure
southbound from preston: 10:40 departure
northbound: 08:22, 14:18 & 19:35 departure
Then the full timetable from 27th July.