Start of the year.
Jan-Feb is normally the coldest period.
If heavy frost move all 'protect from frost' plants under cover, I have an area that is covered by green netting. This is next to the house and the two factors prevent the temp getting too low.
Any fine branched trees like Maple and Chinese Elm will loose fine branches if they get too cold.
As Dec. Jan/Feb is when we may get snow. Check that it is not too heavy on the trees as this can break branches. However do not completely remove as this acts as a good insulator.
Time to start preparing for spring.
What trees will need new pots, how are your supplies of soil, mesh grit etc?
Trip to local bonsai stockist to replenish as necessary.