Extract first trading days of the month

Hi! So I need to extract the trading data of the first day of the month but I don't know how to do it because some months start at day 4 and compare one day with the following one (when decreasing 31st to 1st) we reach a new month

what I had was

days_3 = data(((days>=1) & (days>28)),4)

I know it's not correct cause its just giving trading rates of days 29 30 31 of some months

Answer 1

[Stephen Jue]

What you can do instead is:

days_3 = data((days-[100; days(1:end-1)])<0,4)

This checks the difference between each successive value. When the value decreases, it signifies the start of a new month.