you may need to amend the formula if your regional settings are set to accept dates differently
To count the number of times 27 November 2018 appears in column A, use this
=COUNTIF(A:A,"=27/11/2018")
You mention SUMIF, which suggests requirement to sum values in another column for a single date
To sum the values in column B where the date in column A is 27 November 2018
=SUMIFS(B:B,A:A,"=27/11/2018")
To make the formula more flexible, use cells to hold the values for day (F2), month (G2) and year (H2)
=SUMIFS(B:B,A:A,"="&F2&"/"&G2&"/"&H2)
etc