Complex sentence: On 23 June 1940, I. "Gruppe" was moved to a forward airfield at Grandvilliers for in preparations for missions over the Channel Coast, but the following day, all Bf 109s were sent to Wiesbaden, via Brussels, for a thorough maintenance check.
Split into simpler sentences: On 23 June 1940, I. "Gruppe" was moved to a forward airfield at Grandvilliers for in preparations for missions over the Channel Coast. On 24 June, all Bf 109s were sent to Wiesbaden, via Brussels, for a thorough maintenance check.