Blackboard has a feature to help adjust dated items (announcements, assignments, content) for the new term. After you've copied forward your course materials from a previous semester (see Copy or Import Course Content), go under Control Panel to Course Tools/Date Management to change availability and release dates to reflect the current term's dates. This saves entering each announcement/assignment/item etc. to adjust dates.
See Blackboard documentation about Date Management:
Please note: After Blackboard has adjusted all the dates, it will show you a grid of each item with its old and new dates; it will also send an email. If you have announcements that have no date restrictions, but you include them in the date adjustments, you may see some unexpected dates in the feedback on the confirmation web page and email. However, the announcement itself will still be available and without date restrictions, as you would have expected.