Late war sensors "Snorkel Antenna'
 

When is the correct month/year to apply this mod to my sub ?

Instructions for using this mod are to be found in the GWX manual on page 44. This does not however state which year or month, I suspect that sometime after the snorkel antenna was introduced a 'uboat refit' page will appear with the antenna availability when you are at base prior to starting a mission. I think the only way is to constantly check your 'uboat' menu when you are at base.
After another look in the manual, FuMB-3 bali was installed on VII and IX boats from October 1944, FuMB Leros was installed on XXI boats from January 1945 but I do suggest you read the GWX manual as there is other pertinent information for this snorkel anttena mod.

Correction: the instructions start on page48 of the manual.

Can someone explain, how to enable the FuMB37 Leros for the XXI earlier than Jan.'45?