Not sure if you can configure this using a deskphone, but you can by using Callpilot Manager software, on a PC connected to the same network as the BCM50. You can download CallPilot Manager from the BCM50 itself, by opening the IP address of the BCM in your PC's browser (might be
Open CallPilot Manager, click on Auto-Attendant, and scroll down to the relevant line (which will say "auto-attendant" as the answer mode). Click on the table next to this.
Familiarise yourself with this table, which lists 4 different time periods, with the relevant greetings (and trees) which "kick in" at these times, listed above. The greeting and tree which are listed under "non-business" are probably the ones which you would like to use during holidays, assuming this corresponds with 5pm onwards, on the table.
(You can explore what the corresponding greetings and trees are by returning to the main menu and going to "Autoattendant -> Company Greetings", and "Custom Call Routing". You might only have 1 greeting in use, with 2 trees: one for business hours, one for out-of-hours.)
You can now set up holiday dates by going to "Auto-attendant -> Holidays". Click "add", and just choose the same Greeting and Tree which are normally in use overnight, and set them in all 4 time periods so it stays the same all day. It doesn't really matter what the times are set to if you're using the same one all day. Then when the holiday is over, the phone will return to ringing as normal, from the time set in the first column of the table above.
Hope that makes sense - I'm no expert, but I've just done the same with our BCM50.