How to count events on a particular date

[hmsbablu]

I am a newbie to excel and I got some problem in counting the number of events took place on a particular date.

For Excample:

Col A1 contains the date, and Col B1 contains the details of the event. Now If on any single particular date the number of events reaches maximum three then the user should not be allowed to enter the 4th event details. And in addition a message alert box should display.

Kindly help me with some either formula or code.

 

[Oaktree]

Select A1:A100 (or whatever the last cell in your range is).

Go to data --> validation.

Choose to allow --> custom where the formula is =COUNTIF(A$1:A1,A1)<4. Then, click the "Error Alert" tab (in data --> validation) and enter the message you'd like to display if the user is trying to enter a 4th event for that date.

Note that copy & paste is the Achilles' Heel of data -- > validation (as copy & paste will ignore (or more specifically, replace) the validation in the target cell). If this becomes an issue, you'd have to turn to VBA.