They get letters any day. It's great encouragement for them.
BopsMom here is the site explaining when letter can be recieved and when they can write. hope this helps answer your questions.
You are very welcome.
They receive letters throughout the week but theirs only get mailed out on Monday. You will learn which day is "mail day" for you. Typically Wed-Fri depending on how far you live from Great Lakes.
Know that letters mailed before you get the form letter do not get to your recruit much quicker, if at all, than letters mailed after receiving the form letter because the recruits cannot receive mail until they are in their permanent ship and a Recruit Mail Petty Officer has been trained in how to handle mail, which means it is usually sometime in the third week or later
At first recruits are only permitted to write during holiday routine on Sunday and later they may be able to write on one or two other days as well (often Tuesday and Thursday) and some Divisions may earn the privilege of writing every day if they are doing really well or if the RDC feels that providing that privilege will boost morale. Letters are put in the mail bag on Sunday and the RMPO mails them on Monday unless Monday is a federal holiday. Any letters written throughout the week won't go in the mail bag until Sunday during holiday routine. Some RDCs permit letters to be placed in the mail bag only on Sunday afternoon with mail going out Monday morning even when they can write on other days and others permit them to put letters in the mail bag on the day they are written for the RMPO to mail the next morning. It is up to the RDC and there does not seem to be a "norm" for this.
Once your recruit can receive mail, Mail Call is every weekday evening (M-F). All mail received at RTC is distributed to the recruits on the same day that it is processed. The division's RMPO distributes the mail to the recruits during Mail Call in their compartments that evening sometime prior