Can I put resignation notice before going onsite or after coming from onsite?

I work as a software testing engineer at a small IT company. My notice period is 3 months, currently planning to serve notice period. But my company decided to send me onsite client visit, this is not for training only on onsite work.
My question is, can I put resignation notice before going onsite or after coming from onsite? Please suggest me which is better?
In my offer letter nothing mentioned about notice period during onsite.

You can choose the time for submitting resignation.
During the notice period you are like a normal employee obliged to do the work for which you are employed.

Dear Kumar,
Separation from company has nothing to do with the location of the duties. Sending you to work at the site was the administrative requirement and that's what your company has done it. Therefore, go ahead and submit letter of resignation. However, till you complete your notice period, on-site or off-site, accept the duties assigned to you.
Thanks,
Dinesh Divekar

Dear Kumar,
To work or not to work furthermore is an independent decission. The change of place due to assignemt has got nothing do with your resignation. Your three month notice period will count from the date on which you submit your resignation. Your separation from company has nothing to do with the location.
Therefore you take a decission which period is best suit to you for putting resignation. Till completion of notice period you will remain as usual like other employees of the establishment.