The first line in the written file will be the column name. To avoid the conflict with the first line with the "Source Date: 01/01/2020\n" consider setting an attribute.
library(data.table)
cat("Source Date: 01/01/2020\n", file = "dt.csv")
dt <- data.table(a = 1:5)
fwrite(dt, "dt.csv", append =TRUE)
rewound <- fread("dt.csv")
str(rewound)
#> Classes 'data.table' and 'data.frame': 5 obs. of 1 variable:
#> $ Source Date: 01/01/2020: int 1 2 3 4 5
#> - attr(*, ".internal.selfref")=<externalptr>
rewound
#> Source Date: 01/01/2020
#> 1: 1
#> 2: 2
#> 3: 3
#> 4: 4
#> 5: 5
cat("Source Date: 01/01/2020\n", file = "dt.csv")
dt <- data.table(a = 1:5)
fwrite(dt, "dt.csv", append =FALSE)
rewound <- fread("dt.csv")
str(rewound)
#> Classes 'data.table' and 'data.frame': 5 obs. of 1 variable:
#> $ a: int 1 2 3 4 5
#> - attr(*, ".internal.selfref")=<externalptr>
rewound
#> a
#> 1: 1
#> 2: 2
#> 3: 3
#> 4: 4
#> 5: 5
dt
#> a
#> 1: 1
#> 2: 2
#> 3: 3
#> 4: 4
#> 5: 5
attributes(dt) <- list(sourcedate = "01/01/2020")
dt
#> [[1]]
#> [1] 1 2 3 4 5
#>
#> attr(,"sourcedate")
#> [1] "01/01/2020"