A full 45 hour rest period which straddles 00:00 Monday can be applied to either the preceding or the following week.
Last weekend I had a 45 hour rest period, as I had done for several weeks beforehand.
This weekend, after working Monday-Friday I worked on Saturday (today) and will be taking a reduced rest.
Next weekend, after working Monday-Friday I will be working on Saturday and will be taking a reduced rest.
The following weekend I will be taking a 45 hour rest period. Therefore I will have complied with the requirement to take a full rest period in every other fixed week.