Transforming data with repeated date headers

First, let's see how we could handle this if we take the first four rows of the sample data. That is, start from this:

From here, we can transpose the table and promote the headers to get precisely the shape you specified.

Here's the code for this simplified example:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCi5JTEtT8HRR0lEy1Dcw0jcyMDIGso2Q2MZIbBMktimcHasTrWRobGIIFDQHYgsgNoOzQZLm5kBpDBkQG6zVyNjM0AhJygDkAojmWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t, Column4 = _t, Column5 = _t, Column6 = _t]),
    #"Transposed Table" = Table.Transpose(Source),
    #"Promoted Headers" = Table.PromoteHeaders(#"Transposed Table", [PromoteAllScalars=true])
in
    #"Promoted Headers"

Now we just need to do the same thing for each subtable. The trick is to use Table.Split to split the table into a list of subtables, transform each table in the list just like above, and then combine the list of tables back into a single table.

Here's the code for this:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("jZGxDoMwEEN/BWVGIr5Aku5dmDtGDF34gv6/eqkqMLobGCyesCL57NbC6/Pe92F9hjFgijJJlKQsxIl4Jl4O3sYWkGboz6KqqnxwN0tR2zidf08lZQhZsSc4Hg9KVt2h8JmCFeJK/CBG9NJHIy/9+b2kr0a304O7B5cPbh9cP9z+OUA2Fywk5wI+/Zzn3gU8AHgB8ATgDcTd4Jre28C67gZ/e/sC", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t, Column4 = _t, Column5 = _t, Column6 = _t]),
    #"Filtered Rows" = Table.SelectRows(Source, each ([Column1] <> " ")), 
    #"Split Transpose Combine" = Table.Combine(
        List.Transform(
            Table.Split(#"Filtered Rows", 4),
            each Table.PromoteHeaders(Table.Transpose(_))
        )
    )
in
  #"Split Transpose Combine"

Personally, I'd recommend unpivoting the Staff ID too so that your final result looks like this:

This format should be easier to work with if you plan to build a report off of it.

Extra steps applied:

#"Renamed Columns" = Table.RenameColumns(#"Split Transpose Combine",{{"Staff ID", "Date"}}),
#"Unpivoted Other Columns" = Table.UnpivotOtherColumns(#"Renamed Columns", {"Date"}, "StaffID", "Value")