I dont think your BIL and DN were bad sports to RSVP no- they may have already made plans months ago to attend a Super Bowl event, not just sit and home and watch. If the party is not on your DS actual birthday, then I can see why they may have made other plans not really considering that they party would be on Sunday.
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For some people (not myself and my family) Super Bowl is an celebration of family & friends, more than football- more like a tradition. Just because you do not celebrate, does not mean it may not be important to other people and/or that they may make plans far in advance.Â
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It is neither bad nor good- just different lifestyle interests.
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I would consider reschedule depending on WHO you invited. Are they mostly like-minded families (not sports oriented like yourself), or heavy sports fans?? Â If most of the invitees are not sports fan, proceed with the party and have fun! If you think few people will attend due to other commitments- call them all personally and try to move it to Sat. Â or the following week. But I would attempt to do so quickly if you want to reschedule.
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As for ruining DS birthday- no you would not ruin it. If you reschedule the party - make it  a special day anyway and do something really fun. Then when you do have the party , have fun. Or if you do have it on Super bowl Sun- do you think he will have 'less' of a good time if fewer people come?? I know my 5 yr olds would not notice it much- they would have fun no matter what if we had decorations & a cake!
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FWIW- I did not know it was Super Bowl Sun until yesterday.
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