I've seen people talking about they got paid on certain days, and buying was at XX.XX, and someone else got paid days earlier and it was at XX.XX. How do you control something like that, You can't control what day you get paid, that's up to your agency, but they are consistent and predictable in my experience. Look at your paysheet or your bank statement, it will tell you what date you last got paid, then figure the next paydates will be the same day of the week, 2-week intervals-assuming you get paid biweekly.
Can you always switch your purchases, YES, you can. Get familiar with your agency's procedures for making changes to your pay and TSP allocations. HR dept. is your first stop for getting questions answered.
If they've set you up with a self-service website for these things, you'll need to get set up with a password for access if you don't have one yet. Hope I'm not insulting you with such basics. From the questions, hard to tell.
and when would you do that to time it for your next contribution?