Asked and answered here: 

https://www-secure.symantec.com/connect/forums/calender

Please do not ask 'what if' questions.

You can answer question 1 and 2 yourself by looking at your own environment. 
Tell us what you see?

in my environment  calender scheule was tue and wed checked and window was from 8 in night(tue) to 8 in moning(wed)--Spanning midnight.Retry was not checked.

Backup was started at 8 and completed till 11 in night.

and again backup started at 12 night on wed.Why?

Please find this topic in Admin Guide I: About schedule windows that span midnight

Read the entire section, then come back and tell us why backup ran again after midnight....

I read this guide.

One last question:

But what if i would only checked tue and not wed and my window is same as above.Then?(8 in night(tue) to 8 in moning(wed)-

Read the next bit in the manual ..... you could essentially say that your schedule is an 'open schedule' that only runs for 12 hours & not the full week.

On second thoughts, to make it even simpler, per your quote:

what if i would only checked tue and not wed

....when do you think your calendar schedule will run??

Backup job generated on Tuesday. Nothing happen Wednesday midnight.

Thanks.

My backup job will run till 12 on tue and not on wed then.

Right?