Poland Visa Type and Travel Deal

Do I need to mention it in the cover letter?

There is no harm in mentioning about the trade fair in your cover letter. You can provide the details about the fair and since you have already registered you can also show a receipt if you have gotten one.

Should I attach my marriage certificate and birth certificate of my kid?

Attaching birth and marriage certificate would put some weight to your application. These documents show your strong ties to the home country and could work as a guarantee that you will return to India after your visit.

You should explain:

• Why you want to go.
  Combining business and tourism is entirely normal, but make sure that you provide a clear premise of the trip. For professional reasons you want to go to the trade show, and while you're in the area you want to do some tourism. You wouldn't go without the trade fair.
• Who pays for your trip, respective how you pay for your trip.
  This is very important. If your employer sends you and pays for it, you have a very compelling reason to go. If you pay for it yourself, you should make clear how it fits into your business finances.
• Your financial and other situation at home, and that you have strong ties to your home.
  The visa official will ask himself or herself "is Roxon going to overstay or return home?" Family who stay behind may help, if it doesn't look as if you want to work in the Schengen area and send the money home.

If you get a Schengen visa for just one day, you could travel across an internal border, apply for ayslum, and start lengthy proceeding to determine if you can stay or get deported -- and from India, there is a minimal chance that you could stay. For that reason the visa for that first day is difficult to get, getting a few days more is comparatively easy if it looks as if you can afford it.