In the Metrics portion of the CDP application, when referencing the "Week Number" of transactions in 2021, there is a "Week 53" at the beginning of January.
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When standardizing dates, it is necessary to define 52 weeks in a year, for accounting purposes. These are generally broken up into four 3 month quarters, where each quarter consists of two 4 week months and one 5 week month. Please reference this Wikipedia article for further details into the specifics of why and how, but the relevant quote is here:
A disadvantage of the 4–4–5 calendar is that it has only 364 days (7 days x 52 weeks), meaning a 53rd week will need to be added every five or six years: this can make year-on-year comparison difficult.