How prior i can submit my application for trv..as i am invited for my nephew's birthday celebration in september 2018...the date is 20 septeber 2018..can i submitt my application in july
Visiting for a nephew's birthday is not considered a good reason for travel. Sounds like you have had at least one TRV refusal this year. I wouldn't recommend applying again unless there has been a significant change in your application. If you have had more than one refusals within the past year you really shouldn't apply again for a while.
You shouldn't keep 'trying your luck'. You should take a break from reapplying. Visa approvals don't work that way. Are you referring to three visa refusals for yourself? Or for your sister? I had mentioned earlier that probably your employment in the Punjab police has something to do with your refusals. I would order GCMS notes to understand why you're being refused. BTW birthday celebrations are not compelling reasons to visit