Is it appropriate to wear new wardrobe, shoes or use new accessories in the month of Muharram under the condition that the new belonging was ordered/purchased before the month started but reached you afterwards? Also, is there any recommendation of any specific days to start wearing/using new purchases on, in other months too?

It is not haram but these are the days of mourning and ahlul bait as were not wearing therefore wait until Rabiul Awwal and as you mentioned that it was ordered before Moharram then In’Sha’Allah wear after Moharram
Best day for wearing new cloth is Friday