pachederm,
Try:
UPDATE tCBLAggregation
SET tCBLAggregation.CBL_1_Date = [qBestof4CBLs_avg].[CBL_x_Date],
tCBLAggregation.CBL_1_kW = [qBestof4CBLs_avg].[AvgOfCBL_x_kW]
From tCBLAggregation, [qBestof4CBLs_avg]
Where tCBLAggregation.[Event Date] = qBestof4CBLs_avg.[Event Start Date] AND...