Date, Action
What it is
YYMMDD on which a requested action (e.g. file update) is to occur.
DE 73 on the wire
Bitmap position
DE 73 is bit 9 of the secondary bitmap: byte 2, mask 0x80. A secondary bitmap with only DE 73 present reads 0080000000000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 73 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 73 is fixed length: always 6 digits, with no length prefix. A parser reads exactly that many and moves on, so shorter values are right-aligned and padded with leading zeros.
Size in the message
At most 6 bytes when the message travels as text (ASCII or EBCDIC), or 3 bytes in binary dialects that pack digits as BCD.
Format n
Format n means numeric: the digits 0–9 only.
Specification reference
Defined in ISO 8583:1987 (base). Card networks publish their own dialect tables (Visa BASE I / V.I.P., Mastercard CIS) that refine formats, lengths, and — for private-use elements — the entire layout. Always confirm against the interface specification you implement.