I use google calendar on android at the top left is 3 little bars tap on it and it opens a drawer. Scroll down and you can check or uncheck holidays and you will see settings/ general in there is location. Sorry don't know what app you have
Go to calendar app in your phone in settings turn on or show holidays and make sure your location is set as Thailand. All public holidays in Thailand will then show
Assuming you are married to a Thai national and want that recognised by Thailand then as above the local district office 'amphur' is where to go.
If you where married outside of Thailand they may ask for the marriage certificate to be certified by your embassy and translated into Thai. If you intend to get married here then you will need a letter from your embassy stating you a free to marry.
Amphur office will issue a certificate and Kor ror 2 document which can then be used at immigration to obtain a stay based on marriage. You will have to show 400K baht in a bank account in your name to obtain a 12 month stay
Last time I stayed on Koh Larn I asked the owner of the hotel if he was going to file and thus I would need to upon returning. He told me that legally they where not a hotel and they didn't have to file, thus I didn't. Crazy inconsistencies