Yes that's possible in Moodle 2.7 with plugin Group Choice installed (not yet released for Moodle 2.8) and using restrict access. Set up the groups in the course and name them for example Group A (Week 3+ 4) and Group B (Week 5-7). Add Group Choice as an activity in the week, where you want to give you students the choice, for example in week 2 and add these groups in Group Choice.
Now you need to set the Restrict access conditions in the weeks, i.e., week 3 - restrict access to group A and close the eye (see screenshot below) so they will not see the access condition nor the activities of that particular week. They will still see the header of the weeks.
Do this for all other weeks, for example for week 5, restrict access to Group B and close the eye.
This worked for me in a Test course in Moodle 2.7.
With kind regards,